We studied the prevalence of Chlamydia trachomatis (CT) infection as well as its predictors among a sample (1000) of visitors (651 male, 349 female) of a clinic for sexually transmitted diseases (STD) in Amsterdam. The prevalence of CT was higher than that of gonorrhoea: among men, 14.3% and 11.5% and among women, 12.9% and 6.3% respectively. Independent predictors for CT infections were for men: number of leucocytes in discharge or urine, nature of urethral discharge, age and sex behavior, and in women: being warned as source or contact of a STD, age, nature of cervical discharge and bleeding from the cervical ostium after insertion of a swab. As many as 92% of the male and 81% of the female visitors of our STD clinic should be examined for CT if the presence of anyone or more of these predictors is taken as a criterion. Even then, 5% of the CT infections in women would still be missed. Based on our study results, material for the diagnosis of CT will from now on be collected routinely from all female patients of our STD clinic as well as from all males with urethritis. We advocate the expansion in the Netherlands of facilities for laboratory CT detection and its standard inclusion in STD examination.