STUDY OBJECTIVE: Hirsutism has negative impact on adolescent psychosocial development for both cosmetic and endocrine reasons. This study evaluated the effectiveness of a new intermittent, low-dose finasteride regimen consisting of 2.5 mg of drug given every 3 days (1 day of treatment, 2 days of drug withdrawal) for 6 months in girls with hirsutism by polycystic ovarian syndrome (PCOS) or idiopathic hirsutism (IH). DESIGN AND PARTICIPANTS: Twenty-eight girls (15-19 y old) with hirsutism were randomly assigned to 2 treatment groups and treated for 6 months. Fourteen patients (7 with IH, 7 with PCOS) received finasteride; fourteen patients (7 with IH, 7 with PCOS) received placebo. Hirsutism score (HS), clinical, and hormonal effects were compared between the 2 groups. RESULTS: In patients treated with finasteride, the HS value at 6 months was 52.9% lower than that observed at baseline in girls with IH, and 52.8% lower in girls with PCOS (P