Doctors in Germany say a patient appears to have been cured of Aids by a bone marrow transplant from a donor who had a genetic resistance to HIV.
The researchers in Berlin said the man, who suffered from leukaemia and Aids, had shown no sign of either disease since the transplant two years ago.
The result is expected to encourage further interest in gene therapy as a treatment for Aids.
So far all efforts to find a cure have been unsuccessful.
Berlin's Charite hospital said the 42-year-old patient was an American living in Berlin who has not been identified.
He had been infected with the human immunodeficiency virus that causes Aids for more than a decade and also had leukaemia.
The clinic said since the transplant, tests on his bone marrow, blood and other organ tissues have all been clean.
"More than 20 months after the successful transplant, no HIV can be detected in the patient," the clinic said in a statement.
But leading HIV researcher, Dr Andrew Badley, of the Mayo Clinic in the American state of Minnesota, has warned a lot more tests are needed to prove the man is clear of HIV.
Roughly one in 1,000 Europeans and Americans have an inherited genetic mutation, which prevents HIV from attaching itself to cells.
Two million people die of Aids every year and the virus is estimated to have infected 33 million people worldwide.