Objective - To determine whether those who are aware of being infected with HIV continue to adopt behaviours that place others at risk of HIV infection. Design - Ongoing survey of current diagnosis of sexually transmitted disease and awareness of HIV infection among patients attending genitourinary medicine clinics. Setting - Six genitourinary medicine clinics in England and Wales (two in London and four outside) participating in unlinked anonymous HIV serosurveillance during 1990-3. Subjects - All attenders having blood drawn for syphilis serology for the first time during the calendar quarter of attendance. Main outcome measures - The proportion of syphilis serology specimens with antibody to HIV-1 detected by unlinked anonymous testing of the residue. The proportion of attenders infected with HIV-1 who remained clinically undetected, and the proportion who had another recently acquired sexually transmitted disease. Results - Of 85 441 specimens tested, 2328 (2.7%) were positive for antibodies to HIV-1. About 30% of these specimens were from attenders whose HIV-1 infection remained clinically undetected. HIV-1 infection was found to coexist with another recently acquired sexually transmitted disease in 651 attenders, of whom 522 were homosexual or bisexual men. Of these, 245 (47%) already knew themselves to be infected with HIV-1. This proportion increased between 1990 and 1993. Conclusions - A considerable proportion of patients infected with HIV-1 are not identified by voluntary confidential HIV testing in genitourinary medicine clinics. Substantial numbers of homosexual or bisexual men attending genitourinary medicine clinics continue to practise unsafe sex despite being aware of their infection with HIV-1.