A physician at the Washington Hospital Center gave new hope to balding Americans yesterday, announcing positive test results on a drug to fight the malady that strikes half the male population by age 50.

A year-long test of the drug minoxidil showed a "cosmetically satisfactory response" in nearly a third of the 96 patients who used it, according to dermatologist Thomas Nigra. He said the drug treatment doubled the number of hairs growing in the target areas on 27 of the tested heads. No one, he said, experienced any hair loss.

Before this, Nigra said, treatment for baldness "has been left to charlatans and quacks, and these people have preyed on the emotional needs and hopes of patients. But we really haven't had anything to offer. Perhaps in the near future we will."

Nigra added that tests are still under way across the country, and a spokesman for the drug's manufacturer, Upjohn, said that even if all goes well, the drug would not be on the market before 1987. Results still must be analyzed and approval of the Food and Drug Administration usually takes two years, the spokesman said.

But if the doctor and the manufacturer were cautious, the test subjects were anything but.

"It's wonderful," said 36-year-old Sal D'Adamo, who has been balding since he was 14 years old. "I had new hair growth from the beginning."

A hospital center spokesman said the drug, in a lotion that is rubbed into the scalp, can be used to fight "pattern baldness," the most common type. It is believed to be hereditary and is characterized by a receding hairline and loss of hair at the crown, said spokesman Donna Arbogast.

Patients must have some fine hairs growing in the balding areas, she said. "A person who is totally bald would not find this useful."

Minoxidil is currently used to treat high blood pressure. As is the case with many new drugs, researchers discovered that it had certain side effects -- on some people it grew unwanted hair. That discovery led to year-long experiments at 28 locations across the country.

In a press conference yesterday, Nigra cautioned that the drug "is a therapy, not a cure. You must continue to use it or your hair will fall out."

That unhappy lesson was learned by some of the test subjects. "A few patients opted to discontinue because they didn't think there'd been a noticeable difference," said Arbogast. "After two months when they began again to lose hair . . . they returned wanting to continue."

The drug is also expensive, said Arbogast. A year's supply currently costs about $1,000, she said.

Of the 91 men and five women who took part in the test, 81 showed some increase in the number of hairs in the target areas, a quarter-sized spot on their scalps, Arbogast said. Nine of the subjects developed side effects, including burning or redness of the scalp, and had to drop out.

D'Adamo, who lives in Falls Church, said he treated the rest of his scalp along with the test area. "I conducted my own study . . . . Friends who don't see me every day noticed a big difference. So did my family in New York."

When the study was announced in February 1983, the hospital center was swamped with 10,000 telephone calls and 2,000 letters -- all from people begging to join up.

Said Arbogast: "Just be sure to say we don't need any new volunteers."