OBJECTIVE: To describe the relationships between HIV-negative Australian gay men's sexual behaviour and differences in age between them and their sex partners. DESIGN: Anonymous computer-assisted internet survey was administered. METHODS: We used multinomial logistic regression to examine factors associated with reported differences in partners' age (10 years older, within 10 years and 10 years younger) among 1476 men. RESULTS: Two-thirds of the men reported their most recent casual sex partners were within 10 years of age to themselves, as were 79.3% of their regular partners. Neither men's own age nor relative differences in age with their partners was associated with the likelihood to engage in unprotected anal intercourse. Among men who reported sex with a regular partner, those with older partners tended to restrict themselves to the receptive position (adjusted ORs (AOR)=2.00; 95% CI 1.02 to 3.92; p=0.044). Among men who reported sex with a casual partner, those with younger partners tended to take the insertive position, both on occasions when a condom was used (AOR=2.42; 95% CI 1.39 to 4.20; p=0.002) and on occasions when a condom was not used (AOR=2.54; 95% CI 1.04 to 6.20; p=0.041). CONCLUSIONS: Age differences between gay men and their sex partners make little difference to the likelihood of engaging in sexual risk behaviour overall with either regular or casual partners. Those whose partners are substantially older than themselves are more likely, however, to take the receptive position during anal intercourse. Where risk of HIV infection among younger men is elevated this could be due to differences in sexual position, rather than differences in unprotected anal intercourse per se, during sex with men who are substantially older than themselves.