The three main objectives of this study were to estimate the proportion of the Norwegian population with experience of homosexual behaviour, to study the degree of change in sexual practices among homosexual men during a 5-year period and to study the degree of change in sexual practices reported by HIV positive homosexual men from before to after awareness of HIV-positivity. The data comes from two questionnaire surveys (in 1987 and 1992) of sexual behaviour in the general population of Norway and a questionnaire study of sexual behaviour before and after awareness of HIV-positivity among HIV-positive homosexual men taking part in a cohort study starting in 1988 (the Oslo HIV cohort study). Estimation of the proportion of subjects with homosexual experience was made as well as a trend analysis of the number of male sexual partners per year, number of intercourses per month, condom use and anal sex. Among men aged 18 to 60 from the general population, 3.8% reported homosexual practice during lifetime and 1.2% during the past 3 years. Among women, the same percentages were 3.1 and 1.0. In the surveys, the number of male partners per year decreased significantly for men with current homosexual experience from a yearly median of 1.0 in 1987 to 0.3 in 1992 (p = 0.02). Among HIV-positives, the number of male partners decreased from a yearly median of 4.3 before to 1.6 after awareness of HIV-seropositivity (p < 0.01). Among HIV-positives, a significant increase in the use of condoms, a decrease in the number of intercourses and a decrease in the frequency of anal sex was found. The results show that some changes in sexual practice may have occurred among homosexual men in general in the period from 1987 to 1992, and that more significant changes may have occurred for HIV-positive men. The present data do not support other findings of a relapse to more unsafe sex, but suggest that there is still a need to keep modifying behaviour in order to stop the spread of HIV among men who have sex with other men.