A retrospective study was undertaken in order to compare the efficacy and safety of azithromycin and doxycycline in the treatment of pneumonias caused by Chlamydia spp. Patients with radiologically confirmed pneumonia and positive complement fixation test for chlamydial infection who were hospitalized in the University Hospital of Infectious Diseases, Zagreb during the years 1989-1992 were reviewed. Among them, 83 were treated with azithromycin, given in a total dose of 1.5 g over 5 days (500 mg once daily at day 1 followed by 250 mg at days 2-5, 60 patients) or 3 days (500 mg once daily, 23 patients). Twenty-two patients were treated with doxycycline (100 mg b.i.d. for 10 days). Treatment groups were comparable with respect to age, sex, and severity of signs and symptoms of illness. All the patients were cured. There were no differences in duration of fever after treatment initiation between patients treated with azithromycin (whether pretreated with beta-lactam antibiotics or not) and doxycycline (p > 0.05). In addition, 3- and 5-day azithromycin courses were equally effective (p > 0.05). Both drugs were well tolerated, and only two patients treated with azithromycin reported nausea. It may be concluded that in the treatment of pneumonias caused by Chlamydia spp. azithromycin is as effective and well tolerated as doxycycline.