OBJECTIVE: To compare the effectiveness of finasteride and flutamide in the treatment of hirsutism in patients with polycystic ovary syndrome (PCOS) and with idiopathic hirsutism. DESIGN: Randomized study. PATIENTS: One hundred and ten hirsute patients were selected: 64 women with PCOS and 46 with idiopathic hirsutism. METHODS: Patients were assigned randomly to receive 5mg finasteride once daily or 250mg of flutamide twice daily, for 12 consecutive months. Hirsutism was evaluated at 12 months of therapy, with the Ferriman-Gallwey score and with measurement of the terminal hair diameters (microm) taken from four different body areas. Blood samples were taken for assessment of endocrine and hematochemical parameters. Side effects were monitored during the treatment. RESULTS: Both finasteride and flutamide induced a significant decrease in the hirsutism scores and hair diameters at the end of 12 months. Finasteride reduced the Ferriman-Gallwey score by 31.4% in the PCOS cases and by 34.2% in the idiopathic hirsutism cases, and hair diameter by 27.0-34.1% in PCOS and by 29.6-37.9% in idiopathic hirsutism. Flutamide reduced the Ferriman-Gallwey score by 56.7% in PCOS and by 50.9% in idiopathic hirsutism, and hair diameter by 50. 3-60.0% in PCOS and by 47.7-56.5% in idiopathic hirsutism. Flutamide did not induce hormone variations, while finasteride increased testosterone levels by 40% in PCOS and by 60% in idiopathic hirsutism and decreased 3alpha-androstanediol glucuronide (3alpha-diolG) by 66.7% in PCOS and by 69.5% in idiopathic hirsutism. No important side effects or changes in the hematochemical parameters were observed with finasteride, while two patients (3.6%) in the flutamide group expressed abnormal transaminase levels after 6 months of treatment. Dry skin also appeared significantly more with flutamide (67.3%) than with finasteride (23.6%). CONCLUSIONS: Both drugs are effective in the treatment of hirsutism but flutamide is more effective than finasteride.