The prevalence and clinical features of Chlamydia trachomatis infection were studied in men with nongonococcal urethritis (NGU), in their female sexual partners, and in other women. Of 92 patients with NGU, 36 (39%) were chlamydia-positive; although all had symptoms of urethritis, fewer than half had evident discharge. Clinical features of chlamydia-positive and chlamydia-negative NGU were indistinguishable. Most female contacts of chlamydia-positive patients with NGU were infected with C. trachomatis; contacts of chlamydia-negative patients with NGU seldom harboured chlamydia. Inflammatory and erosive changes were equally common in the uterine cervix of chlamydia-positive and chlamydia-negative women, although almost half of infected cases had no clinical signs of infection and no specific symptoms. In such women the present unavailability of sensitive screening techniques is a major obstacle to the effective control of chlamydial infection.