BACKGROUND: Human papillomavirus 16 (HPV16) has been causally associated with approximately 70% of anal cancers. This cancer is markedly increasing among homosexual men. There is limited knowledge of the epidemiology and natural history of anal HPV infection in homosexual men. METHODS: Behavioral data and sera for antibodies to HPV16 L1 were collected annually for 1427 HIV-negative and 245 HIV-positive Australian homosexual men. Seroprevalence, seroincidence, and risk factors were calculated. RESULTS: Among HIV-negative men, 25.4% were HPV16 seropositive at baseline compared with 44.3% of HIV-positive men. HPV16 seroincidence was 3.1/100 person-years among HIV-negative men and 1.3/100 person-years among HIV-positive men. Seroincidence among HIV-negative men remained >3% per year until 45 years of age, before declining. In multivariate analyses of data from HIV-negative men, seroprevalent HPV16 was associated with sexual risk behaviors and seropositivity for several viral sexually transmissible infections. Seroincident HPV16 was associated with younger age and unprotected anal intercourse with HIV-positive partners. Among men who predominantly practiced insertive anal intercourse, circumcision was associated with a 57% reduction in seroincident HPV16 (hazard ratio = 0.43, 95% confidence interval: 0.21-0.88, P = 0.021). CONCLUSIONS: HPV16 seroincidence remained common in men until their mid 40s suggesting that vaccination may be protective in sexually active young gay men. Both HPV16 seroprevalence and seroincidence correlated well with markers of higher risk sexual activity, particularly receptive anal sexual practices. An association between circumcision and decreased HPV16 seroconversion in HIV-negative men who preferred the insertive position in anal sex was observed.