The objective of this study was to establish a surveillance programme to monitor high risk sexual behaviour among homosexual and bisexual men socializing and/or using health care services in London. Between November 1996 and January 1997 a brief, self-completed questionnaire was distributed in various commercial venues (bars, clubs, and saunas) and genitourinary medicine (GUM) clinics in inner London. A total of 2482 questionnaires were returned (response rate 75%). A third of men (32%) reported having had unprotected anal intercourse in the previous year and 18% had done so with one or more partners of unknown or different HIV status. High risk sexual behaviour was associated with younger age, having casual partners, and recruitment from GUM clinics. The combination of high levels of both sexual risk behaviour and HIV prevalence demonstrates the need for sustained and innovative health promotion campaigns among homosexual men in London.