Perception of risk has been suggested as an important element of sexual behaviour change among people who engage in behaviours which place them at risk of HIV infection. A study of the relationship between perception of risk of HIV infection and risk-related sexual behaviours was conducted in a genitourinary medicine clinic. The sample comprised 767 patients attending over a 3-month period; data collection was by self-completed questionnaire. A total of 574 questionnaires were suitable for analysis, representing a response rate of 75%. The majority of people in the sample reported behaviours which increased their risk of HIV infection, but only 19% (n = 112) of the sample perceived themselves to be personally 'at risk', despite adequate knowledge of HIV transmission and methods of risk reduction. Significant differences between social class groups were found for knowledge scores, with highest scores among professionals and lowest among unemployed subjects (Kruskal Wallis test chi 2 = 24.6, P less than or equal to 0.001). Increasing age was significantly associated with better knowledge; significantly more young people aged 16-20 years who did not perceive themselves 'at risk' (64%), had lower knowledge scores than older people who did not perceive themselves 'at risk' (41%). Among heterosexuals who reported having sex with other people in addition to their regular partner, 79% did not perceive themselves as 'at risk' of HIV infection, and of these, 64% reported only infrequent condom use with casual sexual partners. Significantly more heterosexual men (67%) than women (44%) reported multiple sex partners.(ABSTRACT TRUNCATED AT 250 WORDS)