In a randomised single blind study, pivampicillin was compared with erythromycin in women with urogenital Chlamydia trachomatis infections. The pivampicillin dosage was 700 mg twice a day and the erythromycin dosage 500 mg twice a day for seven days. Follow up took place on days 7 and 14 after the start of treatment. All 26 women treated with pivampicillin were culture negative for chlamydiae at the first and second follow up visits. All 23 women who received erythromycin were culture negative at the first follow up visit, but one was culture positive at the second follow up visit. Gastrointestinal side effects were recorded in five patients receiving pivampicillin and in nine receiving erythromycin. Two patients receiving erythromycin were withdrawn from treatment because of gastrointestinal disturbances, compared with none receiving pivampicillin.