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

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... p II; n~286). All males underwent a routine venereological examination. Men in group II were more often older ( pv0.0005), of Dutch descent ( pv0.0005) and had more sex partners ( pv0.0005). 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. Key words: homosexual men; human immunodeficiency virus infection; sex behavioural determinants; sexually transmitted diseases prevalence. Dutch and international reports have recently demonstrated dramatically increasing rates of gonorrhoea and syphilis, as well as an increase in sexual risk behaviour among men who have sex with men (MSM) (1 -3). Rotterdam is the second largest city in The Netherlands, with an estimated population of almost 700,000 living in and directly around the city. Few data have been published on sexually transmitted diseases (STD) and HIV prevalence and on sexual risk behaviour of MSM living in Rotterdam (4 -7) . Virtually all available information on sexual risk behaviour and the prevalence of STD and HIV among MSM derives from data on visitors to STD clinics (8 -10). It is almost impossible to recruit unbiased groups of MSM for research purposes, which means that enrolment of study participants is always prone to selection bias. We performed the current study in Rotterdam to estimate the prevalence of STD and HIV in MSM and to analyse demographic and sexual behaviour characteristics. To investigate whether MSM visiting the STD clinic on their own initiative and those participating in a newly started gay cohort were different with regard to demography and sexual behaviour, the data of both groups were compared. Comparison of these different groups of MSM allows for a more general assessment of behavioural indicators and possible risk factors for STD and HIV. METHODS Design and procedures The study took place at the STD clinic of the Department of Dermatology and Venereology, Erasmus MC, Rotterdam, The Netherlands -the main facility in Rotterdam for people to be tested for STD. Persons with STD-related symptoms or sexual risk behaviour can be tested free of charge, and without being referred by a general practitioner. Data are routinely recorded in order to detect changes and trends in the epidemiology of STD at an early stage (4, 5). Group I consisted of MSM who attended the STD clinic on their own initiative between January and December 1999. The data concern only the first visit during this recruitment interval. Group II consisted of MSM who were recruited to participate in the Rotterdam gay cohort study. Former (before 1999) visitors to the STD clinic were informed about the possibility of taking part in the study. Men visiting gay bars and saunas in Rotterdam -informed by trained volunteers -were also asked to join the study. In order to enrol a diversity of men, volunteers also visited so-called 'meeting places' where MSM have (anonymous) sex contacts. Advertisements were placed in local mainstream as well as gay periodicals in order to interest potential participants.