“Frankly, I’d rather take the medicine,” said Dr. Robert C. Gallo, director of the Institute of Human Virology at the University of Maryland School of Medicine, referring to antiretroviral drugs.

Moreover, the chances of finding a donor who is a good tissue match for the patient and also has the rare genetic mutation that confers resistance to H.I.V., the virus that causes AIDS, are extremely small. Nonetheless, the man has been free of the virus for 20 months even though he is not using antiretroviral drugs, and the success in his case is evidence that a long-dreamed-of therapy for AIDS  injecting stem cells that have been genetically re-engineered with the mutation  might work.

The cure was announced Wednesday by Dr. Gero Hütter and Dr. Eckhard Thiel, blood-cancer specialists at Charité Hospital in Berlin. The case was described last week in The Wall Street Journal.

Attempts to use bone-marrow transplants in AIDS treatment have been made since the 1980s. In one case, a patient with both AIDS and lymphoma died of the cancer two months later, but was found to harbor no H.I.V.; it was not known if something in the transplant had protected him.

And in a famous 1995 case, Jeff Getty, a prominent San Francisco advocate for AIDS patients, received bone marrow from a baboon, which is resistant to the human virus. He survived 11 years, but died of AIDS and cancer; the transplant had not protected him but antiretroviral triple therapy had been invented in time to help.