Objective To determine whether HIV could be identified in semen samples during the first few weeks after infection. Design A series of three homosexual men with symptomatic primary HIV-1 infection. Methods Each subject provided a series of semen samples that was examined for HIV-1 by virus culture, polymerase chain reaction (PCR) and transmission electron micrography. Results The first samples obtained for each subject (17, 22 and 24 days following onset of primary HIV-1 infection) were all positive by PCR and negative by viral culture. Of 13 samples obtained during the first 80 days after onset of primary HIV-1 infection and analysed by PCR, 10 were positive. Only one of these samples was virus culture-positive. Four semen samples obtained from two subjects during treatment with zidovudine were PCR-positive. Eight samples were examined for presence of HIV-1 by electron microscopy and one was found to be positive. Conclusions These results indicate that men with HIV-1 infection are potentially infectious through sexual transmission during the first few weeks after infection. The findings emphasize that individuals in all stages of HIV-1 infection should practise safer sex to reduce transmission of HIV-1.