OBJECTIVE:To report changes in sexual behaviour among gay men in Sydney from 1986 to 2003. METHODS:Baseline data from four studies of gay men in Sydney were used: the Social Aspects of the Prevention of AIDS study (1986/87: 91 HIV-positive and 444 HIV-negative men); the Sydney Men and Sexual Health cohort (1993-95: 237 HIV-positive and 910 HIV-negative men); the Health in Men cohort of HIV-negative gay men (2001-03: 1,148 men); the Positive Health cohort of HIV-positive gay men (2001/02: 237 men). Each sample was recruited and interviewed using similar methods. RESULTS:Fewer HIV-positive men had sex with casual partners over time (76.9% in 1986/87 to 63.7% in 2001/02; p = 0.001), but more HIV-negative men had sex with 10 or more casual partners in the previous six months (27.7% in 1986/87 to 37.7% in 2001-03; p = 0.012). The proportions engaging in particular sex practices with casual partners changed over time: anal intercourse without condoms that included ejaculation in the rectum fell from 29.4% among HIV-positive men and 32.4% among other men in 1986/87 to 17.8% and 10.0% respectively in 1993-95 (p = 0.034 and p < 0.001 respectively), but increased to 37.7% and 18.4% respectively in 2001-03 (p-values < 0.001); rimming one's partner increased from 36.8% among HIV-positive men and 17.6% among other men in 1986/87 to 63.6% and 52.3% respectively in 2001-03 (p = 0.001 and p < 0.001 respectively). CONCLUSION:Gay men's sexual behaviour with casual partners has changed over time, perhaps partly in response to HIV and partly as a general expansion of sexual repertoires. These changes have implications for gay men's health.