The present study addresses sociodemographic characteristics, knowledge and attitudes regarding HIV/AIDS, as well as risk behaviour and perception of vulnerability to HIV infection, in the first 295 homosexual and bisexual volunteers (excluding male commercial sex workers and transvestites) taking part in a cohort study in Rio de Janeiro. The sample has a higher socio-economic level than the remainder of the cohort, and than comparable strata of the general population, and a sound knowledge about how to protect themselves against HIV infection. In addition, an association between perception of vulnerability to infection and the practice of unprotected anal sex was observed. An association was also observed between higher levels of formal education and perception of vulnerability; however no association was found with markers for past and recent exposure to sexually transmitted infections (hepatitis B, HbsAG and syphilis). The results of the present study suggest that knowledge and awareness of risk do not easily translate into behaviour change, since significant proportions of the cohort continue to practise unprotected sex with both principal and casual partners, despite their awareness of HIV infection risks as well as preventive measures.