OBJECTIVE: To evaluate the extent to which 'high-risk' sexual behaviour is influenced by awareness of partners' HIV status among gay men. DESIGN: Structured interviews and collection of saliva samples for anonymous linked testing for HIV-1 antibodies. SETTING: Genitourinary medicine clinics and the gay community. SUBJECTS: Men (n = 677) who reported sexual contact with another man in the last 5 years. RESULTS: The majority of respondents (63%) had had an HIV-antibody test. Analysis of data showed that in 15% of the respondents' 1380 partnerships, HIV status was known by both parties. However, the majority of partnerships involved only safe sex. Only 26% of the partnerships in which unprotected penetrative anal sex had occurred involved mutual knowledge of HIV status and was most likely to occur with regular rather than non-regular/causal partners. Logistic regression revealed that this latter association could not be explained in terms of mutual HIV status knowledge. CONCLUSIONS: Despite widespread HIV testing, the majority of gay men engaging in high-risk sex are unaware of their partner's HIV status.