By using commercially available spectrophotometric and immunofluorescent immunoassays, Chlamydia trachomatis antigens were detected in first-void urine (FVU) sediments from 224 men attending a sexually transmitted disease clinic at a frequency of 81.6%-86.8% compared with 86.8% (33/38) positive by urethral swab culture (P less than .05). Endocervical cultures from 228 women attending a gynecology clinic yielded 92.3% (12/13) positive compared with 61.5%-76.9% for urine samples in three antigen-detection assays. Culturing urine from either gender yielded low positivity rates (23.7% for men, 15.4% for women). Defining truly infected patients as positive by culture or by any two of the three antigen tests, all assays were 100% specific. Immunodiagnostic testing of male FVU sediment appears to be a reliable, rapid, nontraumatic method for diagnosing chlamydia infection.