OBJECTIVE: To establish a surveillance mechanism of high risk sexual behaviour among homosexual and bisexual men living, socialising and using services in a central London health authority. DESIGN: Baseline survey for a system of repeatable behavioural surveillance using a self-completed questionnaire delivered by healthcare providers. SETTING: Genitourinary medicine clinics, gay bars, clubs, community groups and a cruising ground in the defined geographical area of a central London health authority. PARTICIPANTS: Five hundred and fifty three homosexual and bisexual men. MAIN OUTCOME MEASURES: Self-reported behaviours including unprotected anal intercourse (UAI), HIV status of unprotected anal intercourse partners, uptake of HIV testing and use of condoms at first time of anal intercourse. RESULTS: Five hundred and sixty questionnaires were returned (response rate 76%) from 553 men. A third (35%) of men surveyed had had UAI in the previous year. Nearly a fifth (19%) of the sample had had UAI with one or more partners of a discordant or unknown HIV status. A total of 343 (63%) men had had an HIV test. The proportion of men using condoms on the occasion of first anal intercourse has risen from 6% before 1980 to 88% after 1993. CONCLUSIONS: We have demonstrated that a surveillance programme to monitor high risk sexual behaviour among homosexual men can be easily established. The results can be employed to assess progress towards risk reduction targets and also inform future policy development. Our baseline data demonstrate that a large proportion of homosexual men are continuing to engage in high risk sexual behaviour, although there is some evidence of improvement in condom use at first anal intercourse over time. There is a need for continuing health promotion with evaluation among homosexual men.