This study was performed to investigate whether men who have sex with men visiting the sexually transmitted diseases clinic, and those participating in a gay cohort were different with regard to demographic characteristics, sexual behaviour and sexually transmitted diseases (STD)/human immunodeficiency (HIV) virus prevalence. Data from men who have sex with men presenting at the sexually transmitted diseases clinic (group I; n = 318) were compared with data from men participating in a cohort (group II; n = 286). All males underwent a routine venereological examination. Men in group II were more often older (p < 0.0005), of Dutch descent (p < 0.0005) and had more sex partners (p < 0.0005). New cases of HIV infection were detected far more often in group I (p = 0.04). Also, urethral gonococcal infection was significantly more prevalent in group I (p = 0.003). Multivariate analyses showed that males presenting at the STD clinic (group I) were at higher risk for urethral gonorrhoea. The higher prevalence of HIV infection in group I was associated with a higher prevalence of recent STD, more concomitant urethral gonorrhoea infections at the time of visit, over 10 sex partners in the previous 6 months, and non-Dutch descent.