One hundred and twenty male patients with signs and symptoms compatible with non-gonococcal urethritis were enrolled in a prospective-randomized study to compare the efficacy and safety of a single oral-dose of 1 g azithromycin and a seven-day course of 100 mg doxycycline twice-daily. Clinical examination and culture samples for Chlamydia trachomatis were performed before and approximately 8, 15 and 35 days after starting treatment. Both treatment groups were comprised of 30 chlamydia-positive patients evaluable for efficacy. The eradication rate of C. trachomatis in baseline-positive patients at the first follow-up visit in the azithromycin group was 96% with one persistent case, and 100% in the doxycycline group. After about two weeks, there were two re-occurrences in the azithromycin group, resulting in a cumulative eradication rate of 90% with three culture-positive cases. The corresponding figure in the doxycycline group was still 100%, but there were leucocytes present in the urethral smear of two patients who later proved to be true culture-positive re-occurrences. After about five weeks, there was an additional re-occurrence in the azithromycin group leading to a cumulative eradication rate of 87%, while two re-occurrences in the doxycycline group gave a cumulative eradication rate of 93%. There was no statistically significant difference in efficacy between the single-dose azithromycin and seven-day course of doxycycline in the treatment of patients with chlamydial urethritis.(ABSTRACT TRUNCATED AT 250 WORDS)