The efficacy and safety of roxithromycin 300 mg once a day was compared with that of erythromycin 500 mg twice a day, both for seven days, in a double blind study of 281 patients (188 men, 93 women) with genitourinary chlamydial infections. At the end of the treatment 251 (89%) patients were evaluable, and at follow up two weeks later 227 (81%) were evaluable. the bacteriological cure rate was close to 100% at the end of both treatment regimens. At follow up 55/75 (73%) evaluable men and 38/39 (97%) evaluable women treated with roxithromycin were chlamydia negative compared with 50/71 (70%) evaluable men and 37/42 (88%) evaluable women treated with erythromycin. Of the 47 who were still chlamydia positive, reinfection could not be excluded in half the men and all the women. Side effects were mainly gastrointestinal and were found in about 15% of patients receiving each treatment, but did not necessitate discontinuing in any case. Roxithromycin seems to be as safe and efficacious as erythromycin in treating chlamydial infections in men and women, and it has the advantage that treatment is by a single daily dose.