Clinical and endocrine evaluations of 39 patients with hirsutism were performed to determine the effectiveness and site(s) of action of an antiandrogenic compound, spironolactone. Treatment with spironolactone at a dose of 200 mg/day resulted in a clear beneficial effect on the quantity and quality of facial hair growth in 19 of 20 patients with moderate to severe hirsutism. Regression of hirsutism in terms of diameter, density, and the rate of facial hair growth was noticeable within two months. The maximal effect was observed at six months and was maintained at 12 months of treatment. Spironolactone was equally effective in reducing hirsutism in women with polycystic ovary syndrome and idiopathic hirsutism. These clinical observations were associated with a prompt and sustained reduction in levels of androgen of ovarian origin without affecting levels of adrenal androgen and cortisol. Apart from diuresis, which was limited to the first few days of treatment, there were no discernible side effects during the one year of this study. Our results indicate that spironolactone is a highly effective and safe agent for the treatment of hirsutism through its inhibitory action on both ovarian androgen secretion and peripheral androgen action.