OBJECTIVES: To investigate associations between gay men's optimism and sexual behaviour in the context of new HIV treatments. METHODS: Cross-sectional surveys (using anonymous, self-completed questionnaires) were conducted in Sydney during February 1998 (n = 2200) and in Melbourne during January 1998 (n = 1891). Gay men were recruited at social and sex-on-premises venues, clinics and fair days/carnivals. RESULTS: In a multivariate analysis, unprotected anal intercourse with casual partners (UAIC) was associated with being recruited at a sex-on-premises venue (rather than a clinic or fair day/carnival), HIV positivity, having been tested for HIV less than 6 months ago (rather than over 2 years ago or never having been tested), and not having a regular partner. Over and above these factors, UAIC was associated with agreement with the statements 'An HIV-positive person who is on combination therapy is unlikely to transmit HIV' and 'I'm less worried about HIV infection than I used to be'. CONCLUSIONS: The data reveal a significant relationship between UAIC and certain aspects of optimism in the context of new HIV treatments. Whereas the direction of causality cannot be specified, there is a clear need for HIV and sexual health education programmes to clarify issues of viral load, new and drug resistant strains of HIV, and other infectious agents.