Twelve percent of 596 men presenting to a sexually transmitted disease clinic had positive urethral cultures for Chlamydia trachomatis, and 53% had microimmunofluorescent antibody to chlamydia. Prevalence of C. trachomatis urethral infection was greater in heterosexual than homosexual men (14% versus 5%; p < 0.01), in men under 20 years of age, and in blacks. Only 10% of men with gonococcal urethral infection lacked symptoms or signs of urethritis, whereas nearly 25% of men with C. trachomatis urethral infection had no signs and symptoms, 33% lacked abnormal numbers of leukocytes on urethral Gram stain, and 50% were identified and treated solely on the basis of a screening culture. The number of newly diagnosed cases found by screening cultures was 1.3 per 100 cultures for gonorrhea but 5.5 per 100 for chlamydial infection. Clinicians appropriately treated 91% of men with gonococcal urethritis on their initial visit before culture results were available versus only 51% of men with chlamydial urethral infection. Asymptomatic urethral infections in men eventually contribute to chlamydial infections in women, and culture screening for their detection appears warranted in high-risk populations.