Twenty-one nongonococcal urethritis patients by Chlamydia trachomatis infection were treated with doxycycline (DOXY, Vibramycin) 200 mg/day orally for 2 weeks. The age of these patients were ranged from 17 to 52. C. trachomatis was eradicated 8(89%) of 9 cases on 3rd day and never reisolated on 7th day after treatment. Ten (83%) of 12 patients on 7th day and any patients on 14th day were not suffered from symptoms after treatment. Urethral discharge was not seen macroscopically, 11 (92%) of 12 patients on 7th day and any patients on 14th day. According to the criteria for urethritis requiring at least 4 WBC per high power field in urethral smear and/or first voided urine, the overall clinical efficacy rate was 44% (4/9) on 3rd day, 83% (10/12) on 7th day and 100% (9/9) on 14th day. No side effects were recognized in any cases. In conclusion, DOXY was thought to be useful and safe drug in the treatment of nongonococcal urethritis by C. trachomatis.