The Sydney Men and Sexual Health (SMASH) project is a long-term study of a cohort of gay and bisexual men. It has two study arms: a socio-behavioural arm which focuses on the impact of HIV on the sexual and social lives of the men in the study; and a clinical arm which focuses on serological testing and assessments of disease progression. The project was initially funded through a Commonwealth AIDS Research Grant but the funding has been provided by the NSW Health Department since 1998. It is jointly administered through the National Centre in HIV Epidemiology and Clinical Research, the National Centre in HIV Social Research (both at the University of New South Wales), and the AIDS Council of New South Wales. The study allows socio-behavioural data to be compared with clinical data, while the cohort itself provides a basis to explore particular issues.