Five percent of 145 HIV-1 infected men enrolled in an assisted reproductive technology (ART) program harbored detectable HIV-1 RNA in semen, although they had no other sexually transmitted disease and their blood viral load was undetectable for at least 6 months under antiretroviral treatment. This result justifies measuring HIV-1 RNA in semen before the ART process and suggests that a residual risk of transmission has to be mentioned to the patients who would like to have unprotected sexual intercourse.