PIONEER MID CAP FUND
N-30D, 1998-05-18
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<PAGE>




                                 [PIONEER LOGO]





  PIONEER
  MID-CAP
  FUND


SEMIANNUAL REPORT 3/31/98

<PAGE>
 
    TABLE OF CONTENTS
   --------------------------------------------------------------------
 
<TABLE>
      <S>                                                          <C>
      Letter from the Chairman                                      1

      Portfolio Summary                                             2

      Performance Update                                            3

      Portfolio Management Discussion                               6

      Schedule of Investments                                       9

      Financial Statements                                         14

      Notes to Financial Statements                                20

      Report of Independent Public Accountants                     24

      Trustees, Officers and Service Providers                     25

      Programs and Services for Pioneer Shareowners                26

      Retirement Plans from Pioneer                                28
</TABLE>

<PAGE>
 
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
     LETTER FROM THE CHAIRMAN 3/31/98
   -----------------------------------------------------------------------------
 

     DEAR SHAREOWNER,
   -----------------------------------------------------------------------------
 
     I am pleased to introduce this semiannual report for Pioneer Mid-Cap
     Fund, covering the six months ended March 31, 1998. On behalf of your
     investment team, I thank you for your interest and this opportunity
     to comment on today's investing environment.
 
     The United States' economic news continued to be positive, propelling
     the domestic stock market to yet another series of record highs.
     Large-capitalization stocks were the big winners, with the Dow Jones
     Industrial Average returning nearly 12% as concerns about the Asian
     crisis drew investors toward the safety of U.S. "blue chip"
     companies.
 
     While the mid-sized stocks of the type found in your Fund have not
     kept up with larger stocks, history tells us that markets are
     cyclical. While it is impossible to predict the direction of the
     markets with any degree of certainty, the fact remains that large
     company stocks' record-breaking pace has pushed that group's average
     price-to-earnings ratio to an all-time high. Smaller stocks, on the
     other hand, are trading at lower ratios. It would not be surprising
     to see a new trend emerge, with small- and mid-capitalization issues
     taking leadership positions as investors search for more reasonable
     valuations.
 
     We believe that by using a disciplined approach to choosing stocks -
     purchasing stocks for the right reasons at reasonable prices - we can
     provide the best opportunity for positive long-term results.
     Investors who maintain a diversified portfolio with a long-term
     perspective should be able to weather any short-term risks the market
     may pose.
 
     I encourage you to read on to learn more about Pioneer Mid-Cap Fund.
     If you have questions, please contact your investment professional,
     or Pioneer at 1-800-225-6292.
 

   Respectfully,


   /s/ John F. Cogan, Jr.

   John F. Cogan, Jr.,
   Chairman and President
 
                                                                               1

<PAGE>
 
     PIONEER MID-CAP FUND
   -----------------------------------------------------------------------------
     PORTFOLIO SUMMARY 3/31/98
   -----------------------------------------------------------------------------


     PORTFOLIO DIVERSIFICATION
   -----------------------------------------------------------------------------
    (As a percentage of total investment portfolio)
 

                                  [PIE CHART]
 
 

<TABLE>
                      <S>                            <C>
                      U.S. Common Stocks             99.5%
                      Short-Term Cash Equivalents     0.5%                  

</TABLE>


    SECTOR DISTRIBUTION
   -----------------------------------------------------------------------------
    (As a percentage of equity holdings)
 

                                  [PIE CHART]

<TABLE>

                      <S>                            <C>
                      Technology                     27%
                      Consumer Cyclicals             23%
                      Financial                      20%
                      Healthcare                     10%
                      Energy                          7%
                      Capital Goods                   5%
                      Communication Services          4%
                      Consumer Staples                4%

</TABLE>
 
 
     10 LARGEST HOLDINGS
   -----------------------------------------------------------------------------
    (As a percentage of equity holdings)
 
<TABLE>
      <C>  <S>                          <C>    <C>  <C>                          <C>
       1.  HBO & Co.                    2.74%   6.  Compuware Corp.              2.33%
       2.  Williams-Sonoma, Inc.        2.73    7.  Sterling Commerce, Inc.      2.19
       3.  AccuStaff, Inc.              2.61    8.  Bed Bath & Beyond Inc.       2.18
       4.  Global Industries, Ltd.      2.56    9.  LCI International, Inc.      2.18
       5.  Parametric Technology Co.    2.52   10.  Cendant Corp.                2.10
</TABLE>
 
     Fund holdings will vary for other periods.

 
  2

<PAGE>
 
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
     PERFORMANCE UPDATE 3/31/98                               CLASS A SHARES
 
    SHARE PRICES AND DISTRIBUTIONS
 
   -----------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
      NET ASSET VALUE
         PER SHARE            3/31/98      9/30/97
<S>                          <C>        <C>            <C>
                             $21.53     $23.39
</TABLE>
 
<TABLE>
<CAPTION>
  DISTRIBUTIONS PER SHARE     INCOME     SHORT-TERM      LONG-TERM
    (9/30/97 - 3/31/98)      DIVIDENDS  CAPITAL GAINS  CAPITAL GAINS
<S>                          <C>        <C>            <C>
                                 -            -        $3.354
</TABLE>
 
    INVESTMENT RETURNS
 
   -----------------------------------------------------------------------------
   
 [boxed text]
  
                   AVERAGE ANNUAL TOTAL RETURNS
                   (As of March 31, 1998)


<TABLE>                                             
<CAPTION>
                               NET ASSET   PUBLIC OFFERING         
                   PERIOD        VALUE         PRICE            
                   <S>           <C>           <C>                      
                   10 Years      13.66%        12.99%
                   5 Years       12.24         10.92
                   1 Year        42.97         34.78

</TABLE>

                   * Reflects deduction of the maximum
                   5.75% sales charge at the beginning
                   of the period and assumes reinvest-
                   ment of distributions at net aset
                   value.
            -----------------------------------------------------

The mountain chart on the right shows the growth of a $10,000 investment
made in Pioneer Mid-Cap Fund at public offering price, compared to the
growth of the Standard & Poor's MidCap 400 Index.

[mountain chart]

                              -----------------
                              GROWTH OF $10,000
                              -----------------

<TABLE>
<CAPTION>
                                                  Standard & Poor's 
                      Pioneer Mid-Cap Fund*       MidCap 400 Index

               <S>          <C>                        <C>
               3/31/88       9,425                     10,000
               3/31/89      11,210                     11,705
               3/31/90      12,369                     14,023 
               3/31/91      12,991                     16,874
               3/31/92      15,971                     20,488 
               3/31/93      19,033                     23,791
               3/31/94      20,233                     25,249
               3/31/95      20,756                     27,373
               3/31/96      24,205                     35,150
               3/31/97      23,714                     38,878
               3/31/98      33,904                     57,904

</TABLE>


The Fund adopted its current name and investment objective on February 1, 1996.
Prior to that date, the Fund's name was Pioneer Three and its objective was
 growth and income from a portfolio primarily of small-capitalization stocks.
 
 The Standard & Poor's MidCap 400 Index is an unmanaged measure of 400 domestic
stocks chosen for market size (average capitalization is $2.5 billion),
 liquidity and group representation. Index returns are calculated monthly,
 assume reinvestment of dividends and, unlike Fund returns, do not reflect any
 fees, expenses or sales charges. You cannot invest directly in the Index.
 
 Past performance does not guarantee future results. Return and share price
fluctuate, and your shares, when redeemed, may be worth more or less than their
 original cost.
                                                                               3

<PAGE>
 
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
     PERFORMANCE UPDATE 3/31/98                               CLASS B SHARES
   -----------------------------------------------------------------------------


    SHARE PRICES AND DISTRIBUTIONS
   -----------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
      NET ASSET VALUE
         PER SHARE            3/31/98      9/30/97
<S>                          <C>        <C>            <C>
                             $20.94     $22.98
</TABLE>
 
<TABLE>
<CAPTION>
  DISTRIBUTIONS PER SHARE     INCOME     SHORT-TERM      LONG-TERM
    (9/30/97 - 3/31/98)      DIVIDENDS  CAPITAL GAINS  CAPITAL GAINS
<S>                          <C>        <C>            <C>
                                 -            -        $3.354
</TABLE>
 
    INVESTMENT RETURNS
   -----------------------------------------------------------------------------
[boxed text]   


                   AVERAGE ANNUAL TOTAL RETURNS
                   (As of March 31, 1998)


<TABLE>                                             
<CAPTION>
                                   IF            IF
                   PERIOD         HELD        REDEEMED*   
                   <S>           <C>           <C>                      
                   Life-of-Fund  18.17%        17.02%
                   (2/1/96)      
                   1 Year        41.12         37.12

</TABLE>

                   * Reflects deduction of the maximum
                   applicable contingent deferred sales 
                   charge (CDSC) at the end of the 
                   period and assumes reinvestment 
                   of distributions. The maximum CDSC
                   of 4% declines over six years.
            -----------------------------------------------------
The mountain chart on the right shows the growth of a $10,000 investment
made in Pioneer Mid-Cap Fund, compared to the growth of the Standard &
Poor's MidCap 400 Index.

[mountain chart]
 
                              -----------------
                              GROWTH OF $10,000+
                              -----------------

<TABLE>
<CAPTION>
                                                  Standard & Poor's 
                      Pioneer Mid-Cap Fund*       MidCap 400 Index

               <S>          <C>                        <C>
            2/29/96         10,000                     10,000
            3/31/96         10,254                     10,147
            6/30/96         10,355                     10,440 
            9/30/96         10,765                     10,741 
           12/31/96         11,174                     11,391 
            3/31/97          9,956                     11,224 
            6/30/97         11,079                     12,871
            9/30/97         12,904                     14,938
           12/31/97         11,809                     15,061                         
            331//98         13,750                     16,716 

</TABLE>
 
+ Index comparison begins 2/29/96. The Standard & Poor's MidCap 400 Index is an
  unmanaged measure of 400 domestic stocks chosen for market size (average
  capitalization is $2.5 billion), liquidity and group representation. Index
  returns are calculated monthly, assume reinvestment of dividends and, unlike
  Fund returns, do not reflect any fees, expenses or sales charges. You cannot
  invest directly in the Index.
 
  Past performance does not guarantee future results. Return and share price
  fluctuate, and your shares, when redeemed, may be worth more or less than
  their original cost.


  4

<PAGE>
 
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
     PERFORMANCE UPDATE 3/31/98                               CLASS C SHARES
   -----------------------------------------------------------------------------

 
    SHARE PRICES AND DISTRIBUTIONS
   -----------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
      NET ASSET VALUE
         PER SHARE            3/31/98      9/30/97
<S>                          <C>        <C>            <C>
                             $21.34     $23.33
</TABLE>
 
<TABLE>
<CAPTION>
  DISTRIBUTIONS PER SHARE     INCOME     SHORT-TERM      LONG-TERM
    (9/30/97 - 3/31/98)      DIVIDENDS  CAPITAL GAINS  CAPITAL GAINS
<S>                          <C>        <C>            <C>
                                 -            -        $3.354
</TABLE>
 
    INVESTMENT RETURNS
   -----------------------------------------------------------------------------
   [boxed text]

                   AVERAGE ANNUAL TOTAL RETURNS
                   (As of March 31, 1998)


<TABLE>                                             
<CAPTION>
                                   IF            IF  
                   PERIOD         HELD        REDEEMED              
                   <S>           <C>           <C>                      
                   Life-of-Fund  18.90%        18.90%
                   (2/1/96)      
                   1 Year        42.65         42.65

</TABLE>

                   * Assumes reinvestment of distribu-
                   tions. A 1% contingent deferred
                   sales charge (CDSC) is deducted at
                   the end of the period for redemp-
                   tions made within one year of 
                   purchase.
            -----------------------------------------------------
The mountain chart on the right shows the growth of a $10,000 investment
made in Pioneer Mid-Cap Fund, compared to the growth of the Standard &
Poor's MidCap 400 Index.

[mountain chart]
 
                              -----------------
                              GROWTH OF $10,000+
                              -----------------

<TABLE>
<CAPTION>
                                                  Standard & Poor's 
                      Pioneer Mid-Cap Fund*       MidCap 400 Index

               <S>          <C>                        <C>
             2/29/96        10,000                     10,000
             3/31/96        10,254                     10,147
             6/30/96        10,381                     10,440
             9/30/96        10,795                     10,741
            12/31/96        11,198                     11,391
             3/31/97         9,982                     11,224
             6/30/97        11,215                     12,871
             9/30/97        13,069                     14,938
            12/31/97        11,971                     15,061
             3/31/98        14,240                     16,716 
</TABLE>

 
+ Index comparison begins 2/29/96. The Standard & Poor's MidCap 400 Index is an
  unmanaged measure of 400 domestic stocks chosen for market size (average
  capitalization is $2.5 billion), liquidity and group representation. Index
  returns are calculated monthly, assume reinvestment of dividends and, unlike
  Fund returns, do not reflect any fees, expenses or sales charges. You cannot
  invest directly in the Index.
 
  Past performance does not guarantee future results. Return and share price
  fluctuate, and your shares, when redeemed, may be worth more or less than
  their original cost.

                                                                               5

<PAGE>
 
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
     PORTFOLIO MANAGEMENT DISCUSSION 3/31/98
   -----------------------------------------------------------------------------


    DEAR SHAREOWNER,
   -----------------------------------------------------------------------------

     The past six months were challenging, and at times frustrating, for
     mid-cap investors as large capitalization stocks, like those found in
     the record-setting Standard & Poor's 500 Index, again outperformed
     mid-capitalization stocks. For the first half of its fiscal year, the
     six months ending March 31, 1998, Pioneer Mid-Cap Fund's Class A
     Shares generated a total return of 9.54% at net asset value. In
     comparison, total return was 11.91% for the Standard & Poor's Midcap
     400 Index, and 17.18% for the Standard & Poor's 500 Index.
 
     ASIAN TURMOIL SPARKS SHIFT IN PORTFOLIO
     We saw significant market volatility in the United States in this
     period, sparked when several Asian economies collapsed in late
     October and struggled through early 1998. As the number of imprudent
     loans and investments made in and to Asia over the past few years
     grew apparent, stocks around the world declined as investors became
     wary of the potential global impact on earnings and the possibility
     of a regional or global recession.
 
     When the difficulties surfaced, the Fund held a significant number of
     stocks whose outlook depended upon exports, including exports to
     Asia. In late 1997 and early 1998, we moved out of these, and
     increased our attention to companies that focus on U.S. customers.
     Our judgment was - and remains - that the structural problems in Asia
     are likely to be long-lasting and will reduce foreign demand for U.S.
     products, especially capital goods. In addition, we expect a drop in
     demand from Asia to result in excess inventory and production
     capacity for many globally traded goods. We think this, in turn,
     could significantly drive down prices and cut profit margins for
     manufacturers. As a result, we sold out of trade-sensitive capital
     spending companies such as KLA Instruments and Sun Microsystems.
 
     FOCUSING ON SERVICE PROVIDERS
     We also believe the U. S. economy - particularly consumer
     spending - is strong enough to safeguard against a domestic
     recession, for the near future at least. Given our outlook, in
     November we decided that the most appropriate emphasis for the
     portfolio would

 
  6

<PAGE>
     PIONEER MID-CAP FUND
 
    ----------------------------------------------------------------------------

    ----------------------------------------------------------------------------

     be companies that offer services to our domestic market, rather than
     providing goods or services abroad.
 
     The key to this shift was the difference in pricing between goods and
     services. Although general data show inflation slowing, most service
     sectors have been increasing prices. Trained people are scarce in
     the United States,
     allowing high value-added
     service companies to bump
     up pricing by 3 to 5%
     annually. In contrast,
     manufacturers have often
     had to reduce prices to                      [CHART OMITTED]
     compete with imports. The
     adjacent chart shows that,
     in the United States, the
     service sector has been
     able to sustain or
     increase prices, while
     prices of goods are now
     dropping due to foreign
     competition.
 
     ---------------------------------------------------------------
     CHART:

     PRICES OF GOODS AND SERVICES
     ----------------------------

     <TABLE>
     <CAPTION>
                              GOODS           SERVICES
     <S>                       <C>               <C>
                               2.3%              3.8%
                               2.5               4
                               2.4               4
                               2.6               4.1
                               2.6               4.2
                               2.3               4.2
                               1.9               4.2        
                               1.6               4
                               1.5               4.2
                               1.4               4.2
                               1.9               4.1
                               1.8               3.9
                               2.4               4
                               2.6               3.9
                               2.6               3.8
                               2.4               3.6
                               2.5               3.7
                               2.3               3.6
                               2.6               3.8  
                               2.8               3.8
                               3.2               3.7
                               3.2               3.7
                               2.6               3.8
                               2.8               4
                               2                 3.8
                               1.5               3.6    
                               1.2               3.7
                               1.1               3.7
                               1.1               3.5
                               1.4               3.4
                               1.3               3.1
                               0.9               3
                               0.6               2.9
                               0.2               3.1
                               0.1               2.9
                              -0.1               3.2
                              -0.3               3
</TABLE>                
               Source: International Strategy and Investor
               Group, Bureau of Labor Statistics
     ---------------------------------------------------------------
       
     The Fund has had significant holdings in service industries for some
     time, particularly healthcare and financial service companies that
     focus on the needs of our aging population. In the final months of
     1997 and early in 1998, we added to these sectors. We also added new
     exposure to other service-related industries -- communications, waste
     disposal, business outsourcing, computer services and retail.

 
     IMPROVING PERFORMANCE IN SECOND QUARTER
     Things began to look up for your Fund as the first months of 1998
     unfolded. Domestically focused service businesses generally met or
     exceeded profit expectations, while many manufacturers reported lower
     volumes, prices or profits. The portfolio recovered the losses of the
     fourth quarter, and continued to produce gains through March. The
     largest gains came in computer services, retail and business service
     companies including HBO & Co., AccuStaff, Bed Bath & Beyond and
     Williams-Sonoma. We are encouraged by this show of performance from
     the realigned portfolio.
 
                                                                               7

<PAGE>
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
     PORTFOLIO MANAGEMENT DISCUSSION 3/31/98  (CONTINUED)
   -----------------------------------------------------------------------------

 
     LOOKING FORWARD
     We expect service companies to outpace the broad market for the
     immediate future. However, policy adjustments in Asia appear to be
     moving ahead. Although structural problems of their financial systems
     likely will prevent rapid growth this year, the cumulative effect of
     policy actions may allow Asian economies to show improvement next
     year. If they do, we may again readjust the portfolio to include more
     export-based U.S. companies. Until then, we think the Fund is well
     positioned. We also recognize other risks, such as acceleration of
     the U. S. economy, rising inflation or a change in policy by the
     Federal Reserve.
 
     After all is said and done, we still believe medium-sized,
     high-growth U.S. companies offer some of the most attractive
     aggressive equity investments available today, if viewed with a
     long-term perspective. You should expect periods of short-term
     volatility in today's investment environment, but we believe, and
     history has shown, that perserverance can provide extraordinary
     investment opportunities.
 
    Respectfully,
 

    /s/ Steven C. Carhart

    Steven C. Carhart,
    Portfolio Manager
 

  8

<PAGE>
 
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
     SCHEDULE OF INVESTMENTS 3/31/98
- --------------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
  SHARES                                                                       VALUE
<C>            <S>                                                         <C>
               COMMON STOCKS - 99.5%
               CAPITAL GOODS - 5.0%
               MACHINERY (DIVERSIFIED) - 1.6%
    600,000    AGCO Corp.                                                  $   17,812,500
                                                                           --------------
               MANUFACTURING (DIVERSIFIED) - 1.7%
    389,700    Harsco Corp.                                                $   17,901,844
                                                                           --------------
               WASTE MANAGEMENT - 1.7%
    400,000    USA Waste Services, Inc.*                                   $   17,825,000
                                                                           --------------
               TOTAL CAPITAL GOODS                                         $   53,539,344
                                                                           --------------
               COMMUNICATION SERVICES - 4.7%
               TELECOMMUNICATIONS (LONG DISTANCE) - 3.1%
    600,000    LCI International, Inc.*                                    $   23,100,000
    425,000    Tel-Save Holdings, Inc.*                                         9,668,750
                                                                           --------------
                                                                           $   32,768,750
                                                                           --------------
               TELEPHONE - 1.6%
    750,000    Mobile Telecommunication Technologies Corp.*                $   16,781,250
                                                                           --------------
               TOTAL COMMUNICATION SERVICES                                $   49,550,000
                                                                           --------------
               CONSUMER CYCLICALS - 22.7%
               HOMEBUILDING - 1.5%
    800,000    Clayton Homes, Inc.                                         $   16,200,000
                                                                           --------------
               RETAIL (COMPUTERS & ELECTRONICS) - 1.6%
    250,000    Best Buy Co., Inc.*                                         $   16,656,250
                                                                           --------------
               RETAIL (DEPARTMENT STORES) - 1.2%
    500,000    Saks Holdings, Inc.*                                        $   12,500,000
                                                                           --------------
               RETAIL (DISCOUNTERS) - 1.6%
    400,000    Consolidated Stores Corp.*                                  $   17,175,000
                                                                           --------------
               RETAIL (SPECIALTY) - 11.1%
    400,000    AutoZone, Inc.*                                             $   13,550,000
    500,000    Bed Bath & Beyond Inc.*                                         23,093,750
    400,000    General Nutrition Companies, Inc.*                              15,900,000
    700,000    Office Depot, Inc.*                                             21,787,500
    200,000    Payless ShoeSource, Inc.*                                       15,050,000
    500,000    Williams-Sonoma, Inc.*                                          28,937,500
                                                                           --------------
                                                                           $  118,318,750
                                                                           --------------
</TABLE>
 
   The accompanying notes are an integral part of these financial statements.  9

<PAGE>
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
     SCHEDULE OF INVESTMENTS 3/31/98                             (CONTINUED)
- --------------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
  SHARES                                                                       VALUE
<C>            <S>                                                         <C>
               SERVICES (COMMERCIAL & CONSUMER) - 5.7%
    560,000    Cendant Corp.*                                              $   22,190,000
    400,000    Quintiles Transnational Corp.*                                  19,275,000
    350,000    Stewart Enterprises, Inc.                                       19,468,750
                                                                           --------------
                                                                           $   60,933,750
                                                                           --------------
               TOTAL CONSUMER CYCLICALS                                    $  241,783,750
                                                                           --------------
               CONSUMER STAPLES - 3.9%
               DISTRIBUTORS (FOOD & HEALTH) - 1.3%
    150,000    Cardinal Health, Inc.                                       $   13,228,125
                                                                           --------------
               SERVICES (EMPLOYMENT) - 2.6%
    800,000    AccuStaff, Inc.*                                            $   27,600,000
                                                                           --------------
               TOTAL CONSUMER STAPLES                                      $   40,828,125
                                                                           --------------
               ENERGY - 7.2%
               OIL & GAS (DRILLING & EQUIPMENT) - 5.7%
    300,000    Camco International, Inc.                                   $   18,150,000
    250,000    Cooper Cameron Corp.*                                           15,093,750
  1,330,000    Global Industries, Ltd.*                                        27,098,750
                                                                           --------------
                                                                           $   60,342,500
                                                                           --------------
               OIL & GAS (REFINING & MARKETING) - 1.5%
    400,000    Sun Company, Inc.                                           $   16,350,000
                                                                           --------------
               TOTAL ENERGY                                                $   76,692,500
                                                                           --------------
               FINANCIAL - 19.6%
               BANKS (REGIONAL) - 1.2%
    300,000    SouthTrust Corp.                                            $   12,562,500
                                                                           --------------
               FINANCIAL (DIVERSIFIED) - 3.4%
    575,000    Amresco, Inc.*                                              $   18,831,250
    300,000    Equitable Companies, Inc.                                       16,931,250
                                                                           --------------
                                                                           $   35,762,500
                                                                           --------------
               INSURANCE (LIFE/HEALTH) - 1.6%
    300,000    Conseco, Inc.                                               $   16,987,500
                                                                           --------------
</TABLE>
 
10   The accompanying notes are an integral part of these financial statements.

<PAGE>
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
 
- --------------------------------------------------------------------------------

<TABLE>
<CAPTION>
  SHARES                                                                       VALUE
<C>            <S>                                                         <C>
               INSURANCE (PROPERTY-CASUALTY) - 4.9%
    240,200    Allmerica Financial Corp.                                   $   15,342,775
    200,000    Chubb Corp.                                                     15,675,000
    150,000    Exel Ltd.                                                       11,625,000
    200,000    Partner Re Ltd.                                                  9,825,000
                                                                           --------------
                                                                           $   52,467,775
                                                                           --------------
               INVESTMENT BANKING/BROKERAGE - 0.9%
    231,700    Edwards (A.G.), Inc.                                        $   10,136,875
                                                                           --------------
               INVESTMENT MANAGEMENT - 4.0%
    390,000    Franklin Resources, Inc.                                    $   20,670,000
    315,000    T. Rowe Price Associates, Inc.                                  22,168,125
                                                                           --------------
                                                                           $   42,838,125
                                                                           --------------
               SAVINGS & LOAN COMPANIES - 3.6%
    200,000    Astoria Financial Corp.                                     $   12,362,500
    200,000    Charter One Financial, Inc.                                     13,387,500
    440,000    Washington Federal, Inc.                                        12,210,000
                                                                           --------------
                                                                           $   37,960,000
                                                                           --------------
               TOTAL FINANCIAL                                             $  208,715,275
                                                                           --------------
               HEALTHCARE - 9.8%
               BIOTECHNOLOGY - 1.6%
    700,000    BioChem Pharma Inc.*                                        $   16,931,250
                                                                           --------------
               HEALTHCARE (DRUGS-GENERIC & OTHER) - 1.7%
    500,000    Watson Pharmaceuticals, Inc.*                               $   18,000,000
                                                                           --------------
               HEALTHCARE (HOSPITAL MANAGEMENT) - 2.0%
    750,000    Health Management Associates, Inc.*                         $   21,468,750
                                                                           --------------
               HEALTHCARE (LONG TERM CARE) - 1.9%
    500,000    Integrated Health Services, Inc.                            $   19,656,250
                                                                           --------------
               HEALTHCARE (SPECIALIZED SERVICES) - 2.6%
    200,000    Lincare Holdings Inc.*                                      $   14,125,000
    300,100    Pediatrix Medical Group, Inc.*                                  13,954,650
                                                                           --------------
                                                                           $   28,079,650
                                                                           --------------
               TOTAL HEALTHCARE                                            $  104,135,900
                                                                           --------------
               TECHNOLOGY - 26.6%
               COMMUNICATIONS EQUIPMENT - 1.9%
    700,000    Adtran, Inc.*                                               $   20,781,250
                                                                           --------------
</TABLE>
 
   The accompanying notes are an integral part of these financial statements. 11

<PAGE>
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
     SCHEDULE OF INVESTMENTS 3/31/98                             (CONTINUED)
- --------------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
  SHARES                                                                       VALUE
<C>            <S>                                                         <C>
               COMPUTERS (PERIPHERALS) - 1.8%
    500,000    EMC Corp.*                                                  $   18,906,250
                                                                           --------------
               COMPUTERS (SOFTWARE & SERVICES) - 14.2%
    200,000    BMC Software, Inc.*                                         $   16,762,500
    500,000    Compuware Corp.*                                                24,687,500
    480,000    HBO & Co.                                                       28,980,000
    800,000    Parametric Technology Co.*                                      26,650,000
    500,000    Sterling Commerce, Inc.*                                        23,187,500
    400,000    The Vantive Corp.*                                              14,625,000
    400,000    Wind River Systems*                                             15,900,000
                                                                           --------------
                                                                           $  150,792,500
                                                                           --------------
               ELECTRONICS (COMPONENT DISTRIBUTORS) - 3.5%
  1,300,000    DSP Communications, Inc.*                                   $   22,181,250
    500,000    Gemstar International Group Ltd.*                               15,000,000
                                                                           --------------
                                                                           $   37,181,250
                                                                           --------------
               ELECTRONICS (SEMICONDUCTORS) - 3.4%
    600,000    Analog Devices, Inc.*                                       $   19,950,000
    280,000    Etec Systems, Inc.*                                             16,520,000
                                                                           --------------
                                                                           $   36,470,000
                                                                           --------------
               SERVICES (COMPUTER SERVICES) - 1.8%
    300,000    Fiserv, Inc.*                                               $   19,012,500
                                                                           --------------
               TOTAL TECHNOLOGY                                            $  283,143,750
                                                                           --------------
               TOTAL COMMON STOCKS
               (Cost $755,224,102)                                         $1,058,388,644
                                                                           --------------
</TABLE>
 
12   The accompanying notes are an integral part of these financial statements.

<PAGE>
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------

- --------------------------------------------------------------------------------

<TABLE>
<CAPTION>
 PRINCIPAL
  AMOUNT                                                                       VALUE
<C>            <S>                                                         <C>
               TEMPORARY CASH INVESTMENT - 0.5%
               COMMERCIAL PAPER - 0.5%
$ 4,956,000    Prudential Funding Corp., 5.9%, 4/1/98                      $    4,956,000
                                                                           --------------
               TOTAL TEMPORARY CASH INVESTMENT
               (Cost $4,956,000)                                           $    4,956,000
                                                                           --------------
               TOTAL INVESTMENT IN SECURITIES - 100%
               (Cost $760,180,102)(a)                                      $1,063,344,644
                                                                           --------------
</TABLE>
 
    *  Non-income producing security.
    (a) At March 31, 1998, the net unrealized gain on investments based on
        cost for federal income tax purposes of $760,180,102 was as
        follows:
 
<TABLE>
<S>                                                           <C>
       Aggregate gross unrealized gain for all investments
       in which there is an excess of value over tax cost     $  313,641,505

       Aggregate gross unrealized loss for all investments
       in which there is an excess of tax cost over value        (10,476,963)
                                                              --------------
        Net unrealized gain                                   $  303,164,542
                                                              ==============
</TABLE>
 
    Purchases and sales of securities (excluding temporary cash
    investments) for the six months ended March 31, 1998, aggregated
    $575,893,737 and $643,164,182, respectively.
 
 The accompanying notes are an integral part of these financial statements.  13

<PAGE>
 
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
     BALANCE SHEET 3/31/98
- --------------------------------------------------------------------------------

<TABLE>
  <S>                                                             <C>
  ASSETS:
     Investment in securities, at value (including temporary
       cash investment of $4,956,000) (cost $760,180,102)         $1,063,344,644
     Cash                                                                    378
     Receivables -
        Investment securities sold                                     4,680,759
        Fund shares sold                                                 828,610
        Dividends and interest                                           271,583
     Other                                                                21,086
                                                                  --------------
           Total assets                                           $1,069,147,060
                                                                  --------------
  LIABILITIES:
     Payables -
        Fund shares repurchased                                   $    1,213,393
     Due to affiliates                                                 1,046,165
     Accrued expenses                                                     95,137
                                                                  --------------
           Total liabilities                                      $    2,354,695
                                                                  --------------
  NET ASSETS:
     Paid-in capital                                              $  666,368,393
     Accumulated net investment loss                                  (1,880,436)
     Accumulated undistributed net realized gain on
       investments                                                    99,139,866
     Net unrealized gain on investments                              303,164,542
                                                                  --------------
           Total net assets                                       $1,066,792,365
                                                                  ==============
  NET ASSET VALUE PER SHARE:
  (Unlimited number of shares authorized)
     Class A (based on $1,059,578,881/49,213,183 shares)          $        21.53
                                                                  ==============
     Class B (based on $5,533,136/264,289 shares)                 $        20.94
                                                                  ==============
     Class C (based on $1,680,348/78,740 shares)                  $        21.34
                                                                  ==============
  MAXIMUM OFFERING PRICE:
     Class A                                                      $        22.84
                                                                  ==============
</TABLE>
 
14   The accompanying notes are an integral part of these financial statements.

<PAGE>
 
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
     STATEMENT OF OPERATIONS
     FOR THE SIX MONTHS ENDED 3/31/98
- --------------------------------------------------------------------------------
 
<TABLE>
 <S>                                                         <C>              <C>
 INVESTMENT INCOME:
    Dividends                                                $1,566,734
    Interest                                                    501,577
                                                             ----------
          Total investment income                                             $  2,068,311
                                                                              ------------
 EXPENSES:
    Management fees
       Basic fee                                             $3,052,449
       Performance adjustment                                  (974,392)
    Transfer agent fees
       Class A                                                  690,974
       Class B                                                    7,902
       Class C                                                    1,890
    Distribution fees
       Class A                                                  918,900
       Class B                                                   21,805
       Class C                                                    5,099
    Accounting                                                   44,568
    Custodian fees                                              123,432
    Registration fees                                            52,824
    Professional fees                                            28,678
    Printing                                                     18,658
    Fees and expenses of nonaffiliated trustees                  13,430
    Miscellaneous                                                22,457
                                                             ----------
          Total expenses                                                      $  4,028,674
          Less fees paid indirectly                                                (79,927)
                                                                              ------------
          Net expenses                                                        $  3,948,747
                                                                              ------------
             Net investment loss                                              $ (1,880,436)
                                                                              ------------
 REALIZED AND UNREALIZED GAIN (LOSS) ON INVESTMENTS:
    Net realized gain on investments                                          $101,865,821
    Change in net unrealized gain on investments                               (10,530,583)
                                                                              ------------
       Net gain on investments                                                $ 91,335,238
                                                                              ------------
       Net increase in net assets resulting from operations                   $ 89,454,802
                                                                              ============
</TABLE>
 
   The accompanying notes are an integral part of these financial statements. 15

<PAGE>
 
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
     STATEMENTS OF CHANGES IN NET ASSETS
- --------------------------------------------------------------------------------
     FOR THE SIX MONTHS ENDED 3/31/98 AND THE YEAR ENDED 9/30/97
 
<TABLE>
<CAPTION>
                                                             SIX MONTHS
                                                               ENDED          YEAR ENDED
                                                              3/31/98          9/30/97
 <S>                                                       <C>              <C>
 FROM OPERATIONS:
 Net investment loss                                       $   (1,880,436)  $   (3,467,940)
 Net realized gain on investments                             101,865,821      145,154,691
 Change in net unrealized gain on investments                 (10,530,583)      50,131,527
                                                           --------------   --------------
       Net increase in net assets resulting from
          operations                                       $   89,454,802   $  191,818,278
                                                           --------------   --------------
 DISTRIBUTIONS TO SHAREHOLDERS:
 Net realized gain:
       Class A ($3.35 and $1.88 per share, respectively)   $ (147,104,880)  $  (87,842,235)
       Class B ($3.35 and $1.88 per share, respectively)         (627,640)        (268,625)
       Class C ($3.35 and $1.88 per share, respectively)         (146,941)         (30,516)
                                                           --------------   --------------
             Total distributions to shareholders           $ (147,879,461)  $  (88,141,376)
                                                           --------------   --------------
 FROM FUND SHARE TRANSACTIONS:
 Net proceeds from sale of shares                          $   48,472,566   $   59,240,966
 Reinvestment of distributions                                139,942,753       83,823,606
 Cost of shares repurchased                                  (117,642,977)    (205,791,720)
                                                           --------------   --------------
       Net increase (decrease) in net assets resulting
          from fund share transactions                     $   70,772,342   $  (62,727,148)
                                                           --------------    -------------
       Net increase in net assets                          $   12,347,683   $   40,949,754
 NET ASSETS:
 Beginning of period                                        1,054,444,682    1,013,494,928
                                                           --------------   --------------
 End of period (including accumulated net investment loss
   of $1,880,436 and $0, respectively)                     $1,066,792,365   $1,054,444,682
                                                           ==============   ==============     
</TABLE>
 
<TABLE>
<CAPTION>
                                      '98 SHARES    '98 AMOUNT     '97 SHARES    '97 AMOUNT
<S>                                   <C>          <C>             <C>          <C>
 CLASS A
 Shares sold                           2,175,977   $  44,364,402    2,537,678   $  52,927,652
 Reinvestment of distributions         7,889,373     139,247,420    4,247,313      83,544,664
 Less shares repurchased              (5,683,589)   (114,557,934)  (9,688,258)   (199,441,074)
                                      ----------   -------------   ----------   -------------
         Net increase (decrease)       4,381,761   $  69,053,888   (2,903,267)  $ (62,968,758)
                                      ==========   =============   ==========   =============
 CLASS B
 Shares sold                             147,575   $   3,060,985      274,224   $   5,805,282
 Reinvestment of distributions            33,678         579,591       13,072         255,304
 Less shares repurchased                (136,481)     (2,847,138)    (302,811)     (6,113,758)
                                      ----------   -------------   ----------   -------------
         Net increase (decrease)          44,772   $     793,438      (15,515)  $     (53,172)
                                      ==========   =============   ==========   =============
 CLASS C
 Shares sold                              52,242   $   1,047,179       24,401   $     508,032
 Reinvestment of distributions             6,599         115,742        1,204          23,638
 Less shares repurchased                 (12,341)       (237,905)     (11,303)       (236,888)
                                      ----------   -------------   ----------   -------------
         Net increase                     46,500   $     925,016       14,302   $     294,782
                                      ==========   =============   ==========   =============
</TABLE>
 
16    The accompanying notes are an integral part of these financial statements.

<PAGE>
 
PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
FINANCIAL HIGHLIGHTS 3/31/98
- --------------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                SIX MONTHS
                                                  ENDED          YEAR ENDED    YEAR ENDED    YEAR ENDED    YEAR ENDED    YEAR ENDED
CLASS A                                          3/31/98          9/30/97       9/30/96       9/30/95       9/30/94       9/30/93
<S>                                             <C>              <C>           <C>           <C>           <C>           <C>
Net asset value, beginning of period           $    23.39        $   21.12    $    21.48     $   19.92     $   21.12      $  18.03
                                               ----------        ---------    ----------     ---------     ---------       -------
Increase (decrease) from investment
  operations:
   Net investment income (loss)                $    (0.04)      $    (0.08)         0.18    $     0.24    $     0.24    $     0.28
   Net realized and unrealized gain on
  investments                                        1.53             4.23          1.47          2.70          0.32          3.72
                                               ----------       ----------    ----------    ----------    ----------    ----------
      Net increase from investment operations  $     1.49       $     4.15    $     1.65    $     2.94    $     0.56    $     4.00
Distributions to shareholders:
   Net investment income                                -                -         (0.30)        (0.23)        (0.25)        (0.29)
   Net realized gain                                (3.35)           (1.88)        (1.71)        (1.15)        (1.51)        (0.62)
                                               ----------       ----------    ----------    ----------    ----------    ----------
Net increase (decrease) in net asset value     $    (1.86)      $     2.27    $    (0.36)   $     1.56    $    (1.20)   $     3.09
                                               ----------       ----------    ----------    ----------    ----------    ----------
Net asset value, end of period                 $    21.53       $    23.39    $    21.12    $    21.48    $    19.92    $    21.12
                                               ==========       ==========    ==========    ==========    ==========    ==========
Total return*                                        9.54%           21.36%         8.61%        16.24%         2.62%        22.82%
Ratio of net expenses to average net assets          0.82%**+         0.87%+        0.90%+        0.85%+        0.86%         0.84%
Ratio of net investment income (loss) to
  average net assets                                (0.40)%**+       (0.37)%+       0.85%+        1.18%+        1.19%         1.43%
Portfolio turnover rate                               120%**            63%           75%           19%           15%           18%
Average brokerage commission per share         $   0.0586       $   0.0575    $   0.0527             -             -             -
Net assets, end of period (in thousands)       $1,059,579       $1,048,648    $1,008,177    $1,082,154    $1,017,233    $1,019,059
Ratios assuming reduction for fees paid
  indirectly:
   Net expenses                                      0.80%**          0.85%         0.88%            -             -              -
   Net investment income (loss)                     (0.38)%**        (0.35)%        0.87%            -             -              -
</TABLE>
 
 *  Assumes initial investment at net asset value at the beginning of each
    period, reinvestment of distributions, the complete redemption of the
    investment at net asset value at the end of each period and no sales 
    charges. Total return would be reduced if sales charges were taken into 
    account.
 ** Annualized.
 +  Ratio assuming no reduction for fees paid indirectly.

  The accompanying notes are an integral part of these financial statements.
 
                                                                              17

<PAGE>
 
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
     FINANCIAL HIGHLIGHTS 3/31/98
- --------------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
                                        SIX MONTHS
                                          ENDED       YEAR ENDED    2/1/96 TO
CLASS B                                  3/31/98       9/30/97       9/30/96
<S>                                     <C>           <C>           <C>
Net asset value, beginning of period     $ 22.98       $ 21.02       $ 19.28
                                        --------      --------      --------
Increase (decrease) from investment
   operations:
   Net investment income (loss)          $ (0.11)      $ (0.22)      $  0.12
   Net realized and unrealized
      gain on investments                   1.42          4.06          1.78
                                        ---------     ---------     ---------
      Net increase from investment
         operations                      $  1.31       $  3.84       $  1.90
Distributions to shareholders:
   Net investment income                       -             -         (0.16)
   Net realized gain                       (3.35)        (1.88)            -
                                        ---------     ---------     ---------
Net increase (decrease) in net asset
   value                                 $ (2.04)      $  1.96       $  1.74
                                        ---------     ---------     ---------
Net asset value, end of period           $ 20.94       $ 22.98       $ 21.02
                                        =========     =========     =========
Total return*                               8.88%        19.87%         9.88%
Ratio of net expenses to average net
   assets                                   1.85%**+      2.00%+        1.68%**+
Ratio of net investment loss to
   average net assets                      (1.43)%**+    (1.51)%+      (0.26)%**+
Portfolio turnover rate                      120%**         63%           75%
Average brokerage commission per share   $0.0586       $0.0575       $0.0527
Net assets, end of period (in
   thousands)                            $ 5,533       $ 5,045       $ 4,939
Ratios assuming reduction for fees
   paid indirectly:
   Net expenses                             1.80%**       1.96%         1.66%**
   Net investment loss                     (1.38)%**     (1.47)%       (0.24)%**


 *    Assumes initial investment at net asset value at the
      beginning of each period, reinvestment of distributions, the
      complete redemption of the investment at net asset value at
      the end of each period and no sales charges. Total return
      would be reduced if sales charges were taken into account.
 **   Annualized.
 +    Ratio assuming no reduction for fees paid indirectly.
</TABLE>
 
18   The accompanying notes are an integral part of these financial statements.

<PAGE>
 
     PIONEER MID-CAP FUND
 
- --------------------------------------------------------------------------------
     FINANCIAL HIGHLIGHTS 3/31/98
- --------------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
                                        SIX MONTHS
                                          ENDED       YEAR ENDED    2/1/96 TO
CLASS C                                  3/31/98       9/30/97       9/30/96
<S>                                     <C>           <C>           <C>
Net asset value, beginning of period     $ 23.33       $ 21.12       $ 19.28
                                         --------      --------      --------
Increase (decrease) from investment
   operations:
   Net investment income (loss)          $ (0.09)      $ (0.20)      $  0.03
   Net realized and unrealized
      gain on investments                   1.45          4.29          1.93
                                         --------      --------      --------
      Net increase from investment
         operations                      $  1.36       $  4.09       $  1.96
Distributions to shareholders:
   Net investment income                   -             -             (0.12)
   Net realized gain                       (3.35)        (1.88)        -
                                         --------      --------      --------
Net increase (decrease) in net asset
   value                                 $ (1.99)      $  2.21       $  1.84
                                         --------      --------      --------
Net asset value, end of period           $ 21.34       $ 23.33       $ 21.12
                                         ========      ========      ========
Total return*                               8.96%        21.07%        10.18%
Ratio of net expenses to
   average net assets                       1.84%**+      1.91%+        1.96%**+
Ratio of net investment loss to
   average net assets                      (1.41)%**+    (1.43)%+      (0.29)%**+
Portfolio turnover rate                      120%**         63%           75%
Average brokerage commission per share   $0.0586       $0.0575       $0.0527
Net assets, end of period (in
   thousands)                            $ 1,680       $   752       $   379
Ratios assuming reduction for fees
   paid indirectly:
   Net expenses                             1.80%**       1.87%         1.93%**
   Net investment loss                     (1.37)%**     (1.39)%       (0.26)%**

*    Assumes initial investment at net asset value at the
     beginning of each period, reinvestment of distributions, the
     complete redemption of the investment at net asset value at
     the end of each period and no sales charges. Total return
     would be reduced if sales charges were taken into account.
**   Annualized.
+    Ratio assuming no reduction for fees paid indirectly.
</TABLE>
 

 The accompanying notes are an integral part of these financial statements. 19

<PAGE>
 
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
     NOTES TO FINANCIAL STATEMENTS 3/31/98
   -----------------------------------------------------------------------------
 
    1. ORGANIZATION AND SIGNIFICANT ACCOUNTING POLICIES
 
    Pioneer Mid-Cap Fund (the Fund) is a Delaware business trust registered
    under the Investment Company Act of 1940 as a diversified, open-end
    management investment company. The investment objective of the Fund is
    to seek capital growth.
 
    The Fund offers three classes of shares - Class A, Class B and Class C
    shares. Shares of Class A, Class B and Class C each represent an
    interest in the same portfolio of investments of the Fund and have
    equal rights to voting, redemptions, dividends and liquidation, except
    that each class of shares can bear different transfer agent and
    distribution fees and have exclusive voting rights with respect to the
    distribution plans that have been adopted by Class A, Class B and Class
    C shareholders, respectively.
 
    The Fund's financial statements have been prepared in conformity with
    generally accepted accounting principles that require the management of
    the Fund to, among other things, make estimates and assumptions that
    affect the reported amounts of assets and liabilities, the disclosure
    of contingent assets and liabilities at the date of the financial
    statements, and the reported amounts of revenues and expenses during
    the reporting periods. Actual results could differ from those
    estimates. The following is a summary of significant accounting
    policies consistently followed by the Fund, which are in conformity
    with those generally accepted in the investment company industry:
 
    A. SECURITY VALUATION
 
       Security transactions are recorded on trade date. Each day, securities
       are valued at the last sale price on the principal exchange where
       they are traded. Securities that have not traded on the date of
       valuation, or securities for which sale prices are not generally
       reported, are valued at the mean between the last bid and asked
       prices. Securities for which market quotations are not readily
       available are valued at their fair values as determined by, or under
       the direction of, the Board of Trustees. Dividend income is recorded
       on the ex-dividend date and interest income is recorded on the
       accrual basis. Temporary cash investments are valued at amortized
       cost.
 
       Gains and losses on sales of investments are calculated on the
       identified cost method for both financial reporting and federal
       income tax purposes. It is the Fund's practice to first select for
       sale those securi-


  20

<PAGE>
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
 
   -----------------------------------------------------------------------------

       ties that have the highest cost and also qualify for long-term capital
       gain or loss treatment for tax purposes.
 
    B. FEDERAL INCOME TAXES
 
       It is the Fund's policy to comply with the requirements of the
       Internal Revenue Code applicable to regulated investment companies
       and to distribute all of its taxable income and net realized capital
       gains, if any, to its shareholders. Therefore, no federal income tax
       provision is required.
 
       The characterization of distributions to shareholders for financial
       reporting purposes is determined in accordance with federal income
       tax rules. Therefore, the source of the Fund's distributions may be
       shown in the accompanying financial statements as either from or in
       excess of net investment income or net realized gain on investment
       transactions, or from paid-in capital, depending on the type of
       book/tax differences that may exist.
 
    C. FUND SHARES
 
       The Fund records sales and repurchases of its shares on trade date.
       Net losses, if any, incurred as a result of cancellations are
       absorbed by Pioneer Funds Distributor, Inc. (PFD), the principal
       underwriter for the Fund and an indirect subsidiary of The Pioneer
       Group, Inc. (PGI). PFD earned $52,326 in underwriting commissions on
       the sale of fund shares during the six months ended March 31, 1998.
 
    D. CLASS ALLOCATIONS
 
       Distribution fees are calculated based on the average daily net asset
       value attributable to Class A, Class B and Class C shares of the
       Fund, respectively. Shareholders of each class share all expenses
       and fees paid to the transfer agent, Pioneering Services Corporation
       (PSC), for their services, which are allocated based on the number
       of accounts in each class and the ratable allocation of related
       out-of-pocket expense (see Note 3). Income, common expenses and
       realized and unrealized gains and losses are calculated at the Fund
       level and allocated daily to each class of shares based on the
       respective percentage of adjusted net assets at the beginning of the
       day.
 
       Distributions to shareholders are recorded as of the ex-dividend date.
       Distributions paid by the Fund with respect to each class of shares
       are calculated in the same manner, at the same time, and in the same
 
                                                                              21

<PAGE>
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
     NOTES TO FINANCIAL STATEMENTS 3/31/98                       (CONTINUED)
   -----------------------------------------------------------------------------

       amount, except that Class A, Class B and Class C shares can bear
       different transfer agent and distribution fees.
 
    2. MANAGEMENT AGREEMENT
 
    Pioneering Management Corporation (PMC), the Fund's investment adviser,
    manages the Fund's portfolio and is a wholly owned subsidiary of PGI.
    PMC receives a basic fee that is calculated at the annual rate of
    0.625% of the Fund's average daily net assets. The basic fee is subject
    to a performance adjustment up to a maximum of 60.20% based on the
    Fund's investment performance as compared with the Standard & Poor's
    MidCap 400 Index. For the six months ended March 31, 1998, the
    aggregate performance adjustment resulted in a reduction to the basic
    fee of $974,392. For the six months ended March 31, 1998, the net
    management fee was equivalent to 0.425% of average daily net assets.
 
    In addition, under the management agreement, certain other services and
    costs, including accounting, regulatory reporting and insurance
    premiums, are paid by the Fund. At March 31, 1998, $416,184 was payable
    to PMC related to management fees and certain other services.
 
    3. TRANSFER AGENT
 
    PSC, a wholly owned subsidiary of PGI, provides substantially all
    transfer agent and shareholder services to the Fund at negotiated
    rates. Included in due to affiliates is $166,016 in transfer agent fees
    payable to PSC at March 31, 1998.
 
    4. DISTRIBUTION PLANS
 
    The Fund adopted a Plan of Distribution for each class of shares (Class
    A Plan, Class B Plan and Class C Plan) in accordance with Rule 12b-1 of
    the Investment Company Act of 1940. Pursuant to the Class A Plan, the
    Fund pays PFD a service fee of up to 0.25% of the Fund's average daily
    net assets in reimbursement of its actual expenditures to finance
    activities primarily intended to result in the sale of Class A shares.
    On qualifying investments made prior to August 19, 1991, the Class A
    Plan provides for reimbursement of such expenditures in an amount not
    to exceed 0.15%. Pursuant to the Class B Plan and the Class C Plan, the
    Fund pays PFD 1.00% of the average daily net assets attributable to
    each class of shares. The fee consists of a 0.25% service fee and a
    0.75% distribution fee paid as compensation for personal services
    and/or account maintenance services or distribution services with
    regard to Class B and Class C shares.
 
  22

<PAGE>
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
 
   -----------------------------------------------------------------------------

    Included in due to affiliates is $463,965 in distribution fees payable
    to PFD at March 31, 1998.
 
    In addition, redemptions of each class of shares may be subject to a
    contingent deferred sales charge (CDSC). A CDSC of 1.00% may be imposed
    on redemptions of certain net asset value purchases of Class A shares
    within one year of purchase. Class B shares that are redeemed within
    six years of purchase are subject to a CDSC at declining rates
    beginning at 4.0%, based on the lower of cost or market value of shares
    being redeemed. Redemptions of Class C shares within one year of
    purchase are subject to a CDSC of 1.00%. Proceeds from the CDSCs are
    paid to PFD. For the six months ended March 31, 1998, CDSCs in the
    amount of $7,367 were paid to PFD.
 
    5. EXPENSE OFFSETS
 
    The Fund has entered into certain directed brokerage and expense offset
    arrangements resulting in a reduction in the Fund's total expenses. For
    the six months ended March 31, 1998, the Fund's expenses were reduced
    by $79,927 under such arrangements.

 
                                                                              23

<PAGE>
 
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
     REPORT OF INDEPENDENT PUBLIC ACCOUNTANTS
   -----------------------------------------------------------------------------
 
    TO THE SHAREHOLDERS AND THE BOARD OF TRUSTEES OF
    PIONEER MID-CAP FUND:
 
    We have audited the accompanying balance sheet, including the schedule
    of investments, of Pioneer Mid-Cap Fund as of March 31, 1998, and the
    related statement of operations, the statements of changes in net
    assets, and the financial highlights for the periods presented. These
    financial statements and the financial highlights are the
    responsibility of the Fund's management. Our responsibility is to
    express an opinion on these financial statements and financial
    highlights based on our audits.
 
    We conducted our audits in accordance with generally accepted auditing
    standards. Those standards require that we plan and perform the audit
    to obtain reasonable assurance about whether the financial statements
    and financial highlights are free of material misstatement. An audit
    includes examining, on a test basis, evidence supporting the amounts
    and disclosures in the financial statements. Our procedures included
    confirmation of securities owned as of March 31, 1998 by
    correspondence with the custodian. An audit also includes assessing
    the accounting principles used and significant estimates made by
    management, as well as evaluating the overall financial statement
    presentation. We believe that our audits provide a reasonable basis
    for our opinion.
 
    In our opinion, the financial statements and financial highlights
    referred to above present fairly, in all material respects, the
    financial position of Pioneer Mid-Cap Fund as of March 31, 1998, the
    results of its operations, the changes in its net assets, and the
    financial highlights for the periods presented, in conformity with
    generally accepted accounting principles.


 
    ARTHUR ANDERSEN LLP

 
    Boston, Massachusetts
    May 1, 1998

 
  24

<PAGE>
 
     PIONEER MID-CAP FUND
 
   -----------------------------------------------------------------------------
     TRUSTEES, OFFICERS AND SERVICE PROVIDERS
   -----------------------------------------------------------------------------
 
<TABLE>
      <S>                                <C>
      TRUSTEES                           OFFICERS
      John F. Cogan, Jr.                 John F. Cogan, Jr., Chairman and
      Mary K. Bush                         President
      Richard H. Egdahl, M.D.            David D. Tripple, Executive Vice
      Margaret B.W. Graham                 President
      John W. Kendrick                   William H. Keough, Treasurer
      Marguerite A. Piret                Joseph P. Barri, Secretary
      David D. Tripple
      Stephen K. West
      John Winthrop
</TABLE>
 
    INVESTMENT ADVISER
    Pioneering Management Corporation
 
    CUSTODIAN
    Brown Brothers Harriman & Co.
 
    INDEPENDENT PUBLIC ACCOUNTANTS
    Arthur Andersen LLP
 
    PRINCIPAL UNDERWRITER
    Pioneer Funds Distributor, Inc.
 
    LEGAL COUNSEL
    Hale and Dorr LLP
 
    SHAREOWNER SERVICES AND TRANSFER AGENT
    Pioneering Services Corporation
 
                                                                              25

<PAGE>
 
   -----------------------------------------------------------------------------
     PROGRAMS AND SERVICES FOR PIONEER SHAREOWNERS
   -----------------------------------------------------------------------------
 
    Your investment representative can give you additional information on
    Pioneer's programs and services. If you want to order literature on
    any of the following items directly, simply call Pioneer at
    1-800-225-6292.
 
    FACTFONE(SM)
    Our automated account information service, available to you 24 hours a
    day, seven days a week. FactFone gives you a quick and easy way to
    check fund share prices, yields, dividends and distributions, as well
    as information about your own account. Simply call 1-800-225-4321. For
    specific account information, have your 13-digit account number and
    four-digit personal identification number at hand.
 
    90-DAY REINSTATEMENT PRIVILEGE (FOR CLASS A SHARES)
    Enables you to reinvest all or a portion of the money you redeem from
    your Pioneer account - without paying a sales charge - within 90 days
    of your redemption. You have the choice of investing in any Pioneer
    fund, as long as you meet its minimum investment requirement.
 
    INVESTOMATIC PLAN
    An easy and convenient way for you to invest on a regular basis. All
    you need to do is authorize a set amount of money to be moved out of
    your bank account into the Pioneer fund of your choice. Investomatic
    also allows you to change the dollar amount, frequency and investment
    date right over the phone. By putting aside affordable amounts of
    money regularly, you can build a long-term investment - without
    sacrificing your current standard of living.
 
    PAYROLL INVESTMENT PROGRAM (PIP)
    Lets you invest in a Pioneer fund directly through your paycheck. All
    that's involved is for your employer to fill out an authorization form
    allowing Pioneer to deduct from participating employees' paychecks.
    You specify the dollar amount you want to invest into the Pioneer
    fund(s) of your choice.
 

  26

<PAGE>
 
    ----------------------------------------------------------------------------

    ----------------------------------------------------------------------------
 
    AUTOMATIC EXCHANGE PROGRAM
 
    A simple way to move money from one Pioneer fund to another over a
    period of time. Just invest a lump sum in one fund, and select the
    other Pioneer funds you wish to invest in. You choose the amounts and
    dates for Pioneer to sell shares of your original fund and use the
    proceeds to buy shares of the other funds you have chosen. Over time,
    your investment will be shifted out of the original fund. (Automatic
    Exchange is available for originating accounts with a balance of
    $5,000 or more.)

    DIRECTED DIVIDENDS
    Lets you invest cash dividends from one Pioneer fund to an account in
    another Pioneer fund with no sales charge or fee. Simply fill out the
    applicable information on a Pioneer Account Options Form. (This
    program is available for dividend payments only; capital gains
    distributions are not eligible at this time.)
 
    DIRECT DEPOSIT
    Lets you move money into your bank account using electronic funds
    transfer (EFT). EFT moves your money faster than you would receive a
    check, eliminates unnecessary paper and mail, and avoids lost checks.
    Simply fill out a Pioneer Direct Deposit Form, giving your
    instructions.
 
    SYSTEMATIC WITHDRAWAL PLAN (SWP)
    Lets you establish automatic withdrawals from your account at set
    intervals. You decide the frequency and the day of the month you want.
    Pioneer will send the proceeds by check to the address you designate,
    or electronically to your bank account. You also can authorize Pioneer
    to make the redemptions payable to someone else. (SWPs are available
    for accounts with a value of $10,000 or more.)
 
                                                                              27

<PAGE>
 
    ----------------------------------------------------------------------
    RETIREMENT PLANS FROM PIONEER
    ---------------------------------------------------------------------- 

    Pioneer has a long history of helping people work toward their
    retirement goals, offering plans suited to the individual investor and
    businesses of all sizes. For more information on Pioneer retirement
    plans, contact your investment professional, or call Pioneer at
    1-800-622-0176
 
    INDIVIDUAL RETIREMENT ACCOUNT (IRA)
 
    An IRA is a tax-favored account that allows anyone under age 70 1/2
    with earned income to contribute up to $2,000 annually. Spouses may
    contribute up to $2,000 annually into a separate IRA, for a total of
    $4,000 per year for a married couple. Earnings are tax-deferred, and
    contributions may be tax-deductible.
 
    ROTH IRA
 
    The Roth IRA came about as part of the Taxpayer Relief Act of 1997 and
    is available to investors in 1998. Contributions, up to $2,000 a year,
    are not tax-deductible, but earnings are tax-free for qualified
    withdrawals.
 
    401(K) PLAN
 
    The traditional 401(k) plan allows employees to make pre-tax
    contributions through payroll deduction, up to $9,500 per year or 25%
    of pay, whichever is less. Employers may contribute.
 
    SIMPLE (SAVINGS INCENTIVE MATCH PLAN FOR EMPLOYEES)
    401(K) OR IRA PLAN
 
    Businesses with 100 or fewer eligible employees can establish either
    plan; both resemble the traditional 401(k), but with less testing and
    lower administration costs. Employees can make pre-tax contributions
    of up to $6,000 per year, and an employer contribution is required.
 
      Most retirement plan withdrawals must meet specific conditions to
                               avoid penalties.

  28

<PAGE>
 
    ----------------------------------------------------------------------

    ----------------------------------------------------------------------

 
    401(B) PLAN
 
    Also known as a Tax-Sheltered Account (TSA), a 403(b) plan is
    available only to employees of public schools, not-for-profit
    hospitals and other tax-exempt organizations. A 403(b) plan lets
    employees set aside a portion of their salary, before taxes, through
    payroll deduction.
 
    SIMPLIFIED EMPLOYEE PENSION PLAN (SEP)
 
    SEPs let self-employed people and small-business owners make
    tax-deductible contributions of up to 15% of their income. Generally,
    employers must contribute the same percentage of pay for themselves
    and any eligible employees; contributions are made directly to
    employees' IRAs. SEPs are easy to administer and can be an especially
    good choice for firms with few or no employees.
 
    PROFIT SHARING PLAN
 
    Profit sharing plans offer companies considerable flexibility,
    allowing them to decide each year whether a contribution will be made
    and how much, up to 15% of each participant's pay. These plans can
    include provisions for loans and vesting schedules.
 
    AGE-WEIGHTED PROFIT SHARING PLAN
 
    Like traditional profit sharing plans, employer contributions are
    flexible, but age-weighted plans allocate contributions based on both
    age and salary. Age-weighted plans are designed for employers who want
    to maximize their own contributions while keeping contributions to
    employees affordable.
 
    MONEY PURCHASE PENSION PLAN (MPP)
 
    Money purchase plans are similar to profit-sharing plans, but allow
    for higher annual contributions -- up to 25% of pay. MPPs aren't as
    flexible as profit sharing plans; a fixed percentage of pay must be
    contributed each year, determined when the plan is established.
    Businesses often set up both MPPs and profit sharing plans.
 
      Most retirement plan withdrawals must meet specific conditions to
                               avoid penalties.

                                                                              29


<PAGE>


- ---------------------------------------------------------------------------
HOW TO CONTACT PIONEER
- ---------------------------------------------------------------------------


We are pleased to offer a variety of convenient ways for you to contact up
for assistance or information.


CALL US FOR:

ACCOUNT INFORMATION, including existing accounts,
new accounts, prospectuses, applications
and service forms                                      1-800-225-6292

FACTFONE(SM) for automated fund yields, prices,
account information and transactions                   1-800-225-4321

RETIREMENT PLANS INFORMATION                           1-800-622-0176

TELECOMMUNICATIONS DEVICE FOR THE DEAF (TDD)           1-800-225-1997

WRITE TO US:

Pioneer Services Corporation
60 State Street
Boston, Massachusetts 02109

OUR TOLL-FREE FAX                                      1-800-225-4240

OUR INTERNET E-MAIL ADDRESS                            [email protected]
(for general questions about Pioneer only)

VISIT OUR WEBSITE:                                     www.pioneerfunds.com






THIS REPORT MUST BE PRECEDED OR ACCOMPANIED BY A CURRENT
FUND PROSPECTUS.

<TABLE>
<S>                                           <C> 
[PIONEER LOGO] PIONEER FUND DISTRIBUTOR, INC.
               60 STATE STREET                                 0598-5104
               BOSTON, MASSACHUSETTS 02109    [copyright sign] Pioneer Funds Distributor, Inc.
               WWW.pioneerfunds.com           [recycling sign] Printed on Recycled Paper       
</TABLE>


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