The purpose of the investigation was 1) to determine the prevalence of C. trachomatis among young men and women enrolled in military service, 2) to compare enzyme-immunoassay (EIA) of a urethral swab with a sample of first voided morning urine, 3) to determine frequency of earlier venereal diseases (VD) and actual symptoms of urethritis and cervicitis. EIA positive specimens were confirmed by immunofluorescent microscopy. The investigation comprised 831 men and 80 women (17-26 years). The prevalence was 5.7% (95% confidence limits 4.2-7.4%) and 15% (8-25%) for men and women, respectively (p < 0.005, chi 2). The agreement between the results of the urethral swab and the urine sample was low. Sensitivity and positive predictive value for urethral swab was better than for urine. The handling of urine specimens was more laborious and confirmation more difficult. The percentage of actual symptoms among Chlamydia positive men was 6. The frequency of earlier VD was 7.5% and 10% among men and women, respectively. Treatment with tetracycline was effective, which makes control of eradication unnecessary. We conclude that genital chlamydial infection among young asymptomatic people is common and screening would be desirable. We still recommend urethral swabbing as the routine method until simpler and/or more reliable assays for urine specimens are developed.