Two HIV-positive men in the United States who had to undergo bone marrow transplants are no longer showing signs of the virus and have been able to go off their anti-retroviral medications, researchers said Wednesday.

Doctors are remaining cautious about the cases and are not ready to say the men are cured. But they say blood tests have found no traces of the virus four months after one of the men stopped taking his drugs, and almost two months after the other did the same.

Researchers from Harvard University first reported about the men last year at an international AIDS conference in Washington. At that time, the doctors said the men were showing no traces of HIV eight months after receiving bone marrow transplants.

At the time, the men were still on their anti-retroviral medications but have since stopped them and appear to be staying healthy.

"They are doing very well," researcher Timothy Henrich of Harvard Medical School and Brigham and Women's Hospital told the International AIDS Society conference in Malaysia on Wednesday.

He added though: "While these results are exciting, they do not yet indicate that the men have been cured. Only time will tell."

Both men - who don’t want their names used -- developed Hodgkin’s lymphoma, a type of blood cancer, a few years ago. When multiple rounds of chemotherapy failed to cure them, they each decided to undergo bone marrow transplants.

Such transplants are harrowing and mean using chemotherapy or radiation to destroy one’s own bone marrow and then replacing it with tissue-matched marrow from a donor. The donated marrow contains healthy stem cells that doctors hope will then restore the patient’s ability to produce healthy blood.

During the transplant process, both men stayed on their HIV medications. The researchers believe that prevented the virus from reproducing and infecting the new healthy cells.

While no HIV can be detected in the men’s blood, Henrich and his team warned that the virus could still be “hiding” in other organs, such as the liver or spleen, and could return months from now.

If that occurs, the patients will be put back on the drugs, the researchers said.

The research team hopes to continue testing the men's plasma and tissue for at least a year, to give them a clearer picture of the full impact of the transplant on HIV persistence.

Kevin Robert Frost, chief executive of The Foundation of AIDS Research, said while he welcomed the latest report about the two men, he noted that it wouldn’t be feasible to consider bone marrow transplants for everyone who is HIV-positive.

"While stem cell transplantation is not a viable option for people with HIV on a broad scale because of its costs and complexity, these new cases could lead us to new approaches to treating, and ultimately even eradicating, HIV,” he said.

These men are not actually the first to be functionally “cured” of HIV.

Six years ago, a HIV-positive man with leukemia named Timothy Ray Brown received a bone marrow transplant from a donor who happened to be immune to HIV because he carried a very rare double genetic mutation.

Brown stopped using his anti-retroviral medication ahead of the transplant. Not only did the procedure cure Brown’s cancer, his HIV essentially disappeared. He remains healthy to this day and not taken HIV medication since the transplant.