The Health in Men (HIM) cohort is a longitudinal (5-year) study of HIV-negative gay men in Sydney which started in June 2001. It has two arms: a socio-behavioural arm consisting of regular interviews about the impact of HIV on the sexual and social lives of men in the study; and a clinical arm consisting of serological testing for HIV and other sexually transmissible infections. The data collected allow socio-behavioural data to be directly compared with clinical data. Participants are interviewed annually and tested for HIV and other STIs. Up to 500 new men are recruited to the study each year.