A multi-institutional clinical study was performed on the efficacy of Ofloxacin against chlamydial urethritis in males between January and June, 1987. Chlamydia trachomatis was detected in 68 (38.2%) among 178 patients with male urethritis by the antichlamydial FITC monoclonal antibody technic (Micro Trak). Neisseria gonorrhoeae was isolated in 51 patients (28.7%) and C. trachomatis was also detected in 8 of them. Ofloxacin was administered at a dosage of 600 mg divided into 3 doses for 14 days. One hundred and forty-eight patients including 53 with chlamydial urethritis were evaluable for clinical efficacy. C. trachomatis did not disappear in 6 patients (13.6%) and 2 (8.3%) after the therapy for 7 and 14 days, respectively. After 7 days of therapy, both the pathogen and urethral excretions had disappeared from 79.6% of the patients with chlamydial urethritis, and from 84.8% of those with gonorrheal urethritis; and after 14 days of therapy they had disappeared from 94.1 and 90.9% of the patients, respectively.