What's the difference between the stock market and a roller coaster? Sometimes, not much. A roller coaster takes riders up and down and gives them a thrill but doesn't take them anyplace. This year, that's the way it's been in the stock market.

During January, February and March, stock prices dropped across the board. During April, May and June, stock prices began to rise again, most notably in the blue-chip categories.

But when the first half of the year ended Friday, after all the downs and all the ups, stock prices generally were back where they started on Jan. 1.

The Dow Jones industrials, 30 of the nation's top companies, led the parade. The Dow came to rest Friday at 2880.69, up from 2753.20 on Jan. 1, a gain of 4.6 percent.

In two other blue-chip averages, the Standard & Poor's 500 finished the first half with a 1.3 percent gain, while the New York Stock Exchange Composite turned in a flat performance, up 0.22 percent.

The Nasdaq over-the-counter market was up 1.6 percent for the first half while the American Stock Exchange, home to many battered gold stocks, was down 4.4 percent.

The disastrous performance of bank and thrift stocks hurt many of the averages both nationally and locally.

Washington regional stocks continued to take it on the chin, although the second quarter was not as bad as the first quarter. The Johnston, Lemon & Co. index, which measures the performance of 30 blue-chip local stocks, was off 10.5 percent for the first half.

Three mutual funds that invest in area stocks showed mixed results for the half. The best performer was the Southeastern Growth Fund, up 2.2 percent. The Growth Fund of Washington was off 4 percent and the Washington Area Growth Fund lost 6.8 percent.

Here are the results for Washington area stocks trading at more than $2 a share, with Friday's closing price and the percentage of gain or loss for the first six months of 1990.

First, the winners:

1. Alpha 1 Biomedicals Inc. of Washington saw its stock rise sharply when it announced it was part of a joint venture to begin human testing of a potential AIDS vaccine ($4.63, up 261 percent).

2. CACI International Inc. of Arlington, a professional services firm, saw its class A shares begin to move after the death of chairman Herbert W. Karr. Omaha investor Alan S. Parsow has been pressing for a sale of the company ($3.38, up 68.8 percent).

3. Sage Software Inc. of Rockville creates software used with IBM computers. Sage recently closed its 1990 fiscal year with a $3.5 million profit on a record $26.5 million in sales ($14.25, up 60.6 percent).

4. American Management Systems Inc. of Arlington, a computer systems company. AMS shares hit a rough patch in 1989. Now, with new contracts and the help of IBM, which bought 10 percent of AMS, the firm's stock is rising ($17.63, up 56.7 percent).

5. Penril Corp. of Gaithersburg. A big winner in 1989, Penril jumped after the data communications firm took steps to reduce debt ($8.25, up 55.3 percent).

6. PHP Healthcare Corp. of Alexandria serves government agencies, including the Defense Department ($12, up 54.8 percent). PHP is attracting new contracts.

7. Merry-Go-Round Enterprises Inc. of Joppa, Md., runs 623 stores in 37 states that sell clothing for young men and women ($22.50, up 52.9 percent). The firm's share price reflects its dramatic profit growth.

8. American Capital and Research Corp. of Fairfax provides research and management services ($12.25, up 42 percent). The stock leaped after the firm won a five-year, $119.5 million contract to work on the Boston Harbor cleanup.

9. Micros Systems Inc. of Beltsville, which sells electronic cash registers and was a big winner in 1989, continues to show strong gains ($5.25, up 40 percent).

10. Legg Mason Inc. of Baltimore, a regional brokerage firm, has seen profits rise sharply in fiscal 1990, thanks, in part, to mutual fund fees ($16.75, up 35.4 percent).

11. Alex. Brown Inc. of Baltimore operates retail, investment banking and mutual fund divisions ($14.25, up 32.6). Profits have strengthened in the last year.

12. Life Technologies Inc. of Gaithersburg, a biotech company, is considering the sale of its losing molecular diagnostics business ($18, up 30.9 percent).

13. Danaher Corp. of Washington is owned by the Rales brothers, who are completing their takeover of Easco Hand Tools Inc. ($19.88, up 30.3 percent).

14. Allstate Financial Corp. of Arlington is a company that buys commercial accounts receivables. The firm's first-quarter profits rose 68 percent ($9, up 28.6 percent).

15. Vie de France Corp. of McLean. The bakery and restaurant firm has watched its stock fall steadily. Now, the company has reorganized and is searching for a recipe for success ($2.38, up 26.7 percent).

Here are the losers:

1. Washington Bancorporation, parent of National Bank of Washington, has been battered by internal strife, an unsuccessful effort to sell the bank and a default on its commercial paper ($2.56, down 84.5 percent).

2. Perpetual Financial Corp. of Vienna, the largest thrift in Washington, has been caught between the sagging real estate market and demands by regulators for more reserves to cover loans that might go bad ($2.25, down 68.4 percent).

3. Trustbank Savings F.S.B. of McLean ($2.50, down 58.3 percent). Trustbank lost $18.8 million in its recent 1990 fiscal year after increasing loan reserves.

4. Piedmont Federal Corp. of Manassas, a small Virginia thrift, has seen its profits shrink as the depressed real estate market drys up loan production and requires higher reserves ($7, down 50.4 percent).

5. Data Measurement Corp. of Gaithersburg makes industrial measurement systems. The company's profit pictures continues to be erratic. The company lost $2 million in 1989. ($3.25, down 50 percent).

6. MNC Financial Inc. of Baltimore, the region's largest bank, also has been caught in the squeeze between a battered commercial real estate market and closer scrutiny by regulators ($12.75, down 42.4 percent).

7. Harman International Industries Inc. of Washington, which makes high-fidelity equipment, has warned investors that business is soft and it expects profits for the current year to be 5 percent to 7 percent below last year ($11.75, down 42 percent).

8. Investors Financial Corp. of Richmond, parent of Investors Savings Bank, lost $8.9 million in 1989 and has been cutting costs to meet new capital regulations ($2.75, down 38.9 percent).

9. Microlog Corp. of Germantown, the biggest gainer of 1989 (up 245.5 percent), has dropped off the pace. The company blamed losses on falling sales and a new sales staff ($5.75, down 39.5 percent).

10. WorldCorp Inc. of Herndon, a charter-airline operator, reported a $6.3 million first-quarter loss, which included a $2.1 million loss on junk bond investments ($8.13, down 38.7 percent).

11. First Citizens Financial Corp., parent of Citizens Savings Bank of Silver Spring, reported lower first-quarter profits and increased reserves for loan losses ($5, down 38.2 percent).

12. Versar Inc. of Springfield, an environmental services firm, has seen its profits drop as a result of a slowdown in Environmental Protection Agency and Defense Department contracts ($4.50, down 35.7 percent).

13. Ameribanc Investors Group. of Annandale, the parent of Ameribanc Savings Bank, reported a $3 million first-quarter loss caused by problem real estate loans and the need to add to reserves. ($5.25, down 34.4 percent).

14. Insituform East Inc. of Landover, a firm that repairs sewer pipes, has seen its profits fall as higher-margin contracts for fixing large-diameter pipes have diminished ($4.25, down 33.3 percent).

15. United Savings Bank of Vienna, a thrift with a troubled history, has broken off merger talks with Citibank Federal Savings Bank ($2.50, down 33.3 percent).

Meanwhile, one stock not listed was NVR L.P. of McLean, a home builder whose stock price fell below the $2 cutoff used in compiling this list. The firm has seen its profits hurt by the continuing slump in the new home market. Its stock closed at $1.75, down 65 percent for the first half.