OBJECTIVES: The incidence of HIV and STIs increased among men who have sex with men (MSM) visiting our STI clinic in Amsterdam. Interestingly, HIV increased mainly among older (> or =35 years) MSM, whereas infection rates of rectal gonorrhoea increased mainly in younger men. To explore this discrepancy we compared trends in STIs and HIV in a cohort of young HIV negative homosexual men from 1984 until 2002. METHODS: The study population included 863 men enrolled at < or =30 years of age from 1984 onward in the Amsterdam Cohort Studies (ACS). They had attended at least one of the 6 monthly follow up ACS visits at which they completed a questionnaire (including self reported gonorrhoea and syphilis episodes) and were tested for syphilis and HIV. Yearly trends in HIV and STI incidence and risk factors were analysed using Poisson regression. RESULTS: Mean age at enrollment was 25 years. The median follow up time was 4 years. Until 1995 trends in HIV and STI incidence were concurrent, however since 1995 there was a significant (p<0.05) increase in syphilis (0 to 1.4/100 person years (PY)) and gonorrhoea incidence (1.1 to 6.0/100 PY), but no change in HIV incidence (1.1 and 1.3/100 PY). CONCLUSIONS: The incidence of syphilis and gonorrhoea has increased among young homosexual men since 1995, while HIV incidence has remained stable. Increasing STI incidence underscores the potential for HIV spread among young homosexual men. However, several years of increasing STIs without HIV, makes the relation between STI incidence and HIV transmission a subject for debate.