In 2008 the highest number of HIV diagnoses among men who have sex with men (MSM) was recorded. This study proposes to be the first to estimate HIV prevalence and unrecognised infection in a community sample of MSM in Auckland, and identify factors associated with undiagnosed infection. Participants in the 2011 round of the Gay Auckland Periodic Sex Survey (GAPSS), an established anonymous study of MSM attending community venues in Auckland, will be invited to provide an oral fluid specimen while self-completing a questionnaire. Information will be collected on demographics, testing, sexual behaviour, and other risk practices. The oral fluid HIV antibody results will be linked by code to participants' questionnaires but not to individuals directly. Combining biological markers of actual HIV infection with information on testing and risk behaviours from a high-risk sample of MSM will significantly improve the ability of prevention and testing services to target resources.