OBJECTIVE: To evaluate the effects of long-term administration of finasteride on hirsutism score, basal gonadotropin, and androgen secretion in women with idiopathic hirsutism. DESIGN: Randomized single-blinded study. PATIENTS: Eighteen patients with moderate-severe hirsutism were recruited for the study. INTERVENTIONS: Nine hirsute patients received 7.5 mg/d oral finasteride for a period of 9 months whereas the other nine were treated with placebo. Hirsutism score, serum basal gonadotropin, androgens, estrogen, and sex hormone-binding globulin (SHBG) levels were evaluated in all patients before treatment and every 3 months during treatment. RESULTS: After 6 and 9 months of treatment, the hirsutism score improved significantly in the patients receiving finasteride, whereas no significant modifications were observed in patients treated with placebo. The side effects observed were headache and depression of modest entity during the 1st month of treatments, whereas libido did not change. Serum levels of LH, FSH, androstenedione, unbound T, DHEAS, E2, 17 alpha-hydroxyprogesterone, and SHBG did not change during therapy. Hirsute patients treated with finasteride exhibited a marked decrease of dihydrotestosterone and a significant increase of T serum levels from the 3rd and 6th months of treatment, respectively. CONCLUSION: Finasteride decreased the hirsutism score of patients affected by idiopathic hirsutism with few side effects during treatment. No modification of libido was observed.