In a randomised study the clinical efficacy of ciprofloxacin was compared with that of doxycycline administered in two different dosage schemes to male patients suffering from non-gonococcal urethritis. Fourteen days after completion of therapy (day 21) pyuria was absent in 30 of 100 patients in the ciprofloxacin group; Chlamydia trachomatis was isolated from five and Ureaplasma urealyticum from eight patients. In the 100 mg doxycycline group (n = 60) pyuria was absent in 36 patients (60%) and Ureaplasma urealyticum was isolated from six patients on day 21. In the 200 mg doxycycline group (n = 45) pyuria was absent in 18 patients (40%) and Ureaplasma urealyticum was isolated from two patients on day 21. Side-effects were mild and transient in all groups. It is concluded that ciprofloxacin given in a dosage of 1 g for seven days is not effective in the treatment of non-gonococcal urethritis.