Spironolactone, an aldosterone antagonist with antiandrogenic activities, is often used to treat hirsute women. Several investigators have reported beneficial effects of such treatment, but the dosages used and side-effects encountered differ in their reports. To clarify this issue, 26 consecutive premenopausal women with idiopathic hirsutism were prospectively evaluated during spironolactone treatment. Sixteen women initially received 100 mg spironolactone twice daily on days 4-21 of their menstrual cycles. In 11 women (68%), the dosage had to be decreased or the medication discontinued due to side-effects. The major side-effect, metrorrhagia with 14-day cycles, occurred in 9 women (56%). This side-effect, while not dangerous, was intolerable to these women. In one woman, spironolactone was discontinued because of urticaria and in another because of scalp hair loss. Ten women initially received 50 mg spironolactone twice daily on days 4-21 of their menstrual cycles. Two developed metrorrhagia, but no other side-effects were noted. Because of the incidence of metrorrhagia at higher dosages, we recommend that spironolactone be administered cyclically at a starting dosage of 50 mg twice daily on days 4-21 of the menstrual cycle. Alternatively, one may consider adding cyclical estrogen/progesterone therapy to continuous spironolactone therapy.