The clinical effectiveness of doxycycline (DOXY) in the treatment of chlamydial infections was studied by giving it to 14 patients composed of 9 with nongonorrheal urethritis and 5 with cervicitis, all caused by Chlamydia trachomatis. DOXY was given in an oral dose of 100 mg twice daily for 4 to 19 days. The total dosage ranged from 800 to 3,800 mg. The overall clinical efficacy as assessed on the basis of both bacterial response and relief of subjective or objective symptoms was excellent in all of 12 evaluable patients. Two patients for whom we failed to perform postdose bacteriological examinations were excluded from evaluation. No side effect related to dosing of DOXY developed.