['To evaluate the effect of highly active antiretroviral therapy (HAART) on the sexual behaviour of homosexual men, we conducted (i) an ecological study of time trends in sexual behaviour and sexually transmitted diseases; (ii) a HAART-effect study focused on the practice of unprotected anogenital sex.', 'Subjects were participants in the ongoing Amsterdam Cohort Studies (ACS) among homosexual men, initiated in 1984. Data for (i) represented all ACS visits by HIV-1-positive and -negative participants who entered ACS at or below 30 years of age and were followed until 35 years (n = 1062). Data for (ii) represented all ACS visits of HIV-1-positive men from 1992 to 2000 (n = 365), of whom 84 were HAART recipients with at least 2 months of behavioural follow-up.', '(i) After HAART became generally available in July 1996, unprotected sex was practised more frequently and the incidence of gonorrhoea was higher compared to March 1992-June 1996 among HIV-1-negative and -positive men, respectively. (ii) Among HIV-1-positive men, a higher level of unprotected sex with casual partners was observed after HIV-1 RNA became undetectable and CD4 cell counts increased with the use of HAART. Notably, in individuals who did not receive HAART, high HIV-1-RNA levels (above 10(5) copies/ml) were likewise related to unprotected sex with casual partners.'] Does sexual risk behaviour relate to the virological and immunological improvements during highly active antiretroviral therapy in HIV-1 infection?