The purpose of the study was to investigate possible changes in the prevalence of STD and HIV collected at a Dutch STD clinic in the period 1996 to 2000. Age, gender, ethnic background, sexual preference, intravenous drug use and STD or HIV infection in persons attending an STD outpatient clinic were analysed and compared. The prevalence of HIV infection among the clinic visitors remained stable. The prevalence of Neisseria gonorrhoeae and Chlamydia trachomatis infections increased significantly among heterosexual men and heterosexual women. Among homo- and bisexual men a significant increase was seen in chlamydial infections only. Because of the increasing prevalence of gonococcal and chlamydial infections among STD clinic visitors in Rotterdam, more attention should be paid to coordinated preventive activities, such as health education and contact tracing. Further subgroup analyses should be done in order to get more information on risk behaviour in the different groups.