Background: Human herpesvirus 8 (HHV-8) is a common sexually transmitted agent among homosexual men, but there are few Australian data. We aimed to describe the prevalence and risk factors for seropositivity to HHV-8 in Australian homosexual men. Methods: We conducted a prospective cohort study of 179 homosexual men in Sydney Australia in 1992-1998. Detailed data on sexual behaviour was collected annually, and HHV-8 status was determined at the end of the study by an algorithm based on results of an immunofluorescence assay and an enzyme-linked immunoassay to the K8.1 protein of HHV-8. HHV-8 DNA was detected in buffy coats using a nested qualitative PCR. Results: Data on sexual behaviour in at least three interviews and HHV-8 status were available in 174 (97%) of 179 men who agreed to participate. Of these, 31 (18%) were HHV-8 seropositive, and HHV-8 DNA was detected in 5 (16%) of these. The prevalence of HHV-8 infection was much higher in HIV positive (52%) than HIV negative (11%) men (OR 8.60, 95% CI 3.55-20.86). HHV-8 infection was related to more frequent reporting of unprotected receptive anal sex (OR for most frequent versus least frequent category 3.03, 95% CI 1.01-9.03, P trend 0.02), insertive oro-anal sex (OR for most frequent v. least frequent category 3.02, 95% CI 1.15-7.93, P trend 0.02) and receptive oro-anal sex (OR for most frequent v. least frequent category 3.09, 95% CI 1.11-8.60, P trend 0.05) with casual partners. Conclusions: These data are consistent with sexual transmission of HHV-8, but the precise mode of HHV-8 transmission remains unclear. Studies to elucidate the precise mode of sexual transmission of HHV-8 need to focus on potential salivary transmission, and should collect data on the HHV-8 infection and excretion status of the sexual partner.