A randomized double-blind trial of topical minoxidil therapy was carried out on 56 patients with hereditary male pattern baldness. The subjects selected were required to have a discernible balding patch, a minimum of 2.5 cm in diameter on the vertex of the head where the hairs could be counted and photographed. Minoxidil, 1.0 mL, was applied twice a day to the scalp beginning at the balding vertex and spreading centrifugally around the scalp. Cosmetically acceptable hair growth was achieved in 18 patients (32%). The most notable indicators for regrowth of hair were the number of indeterminate hairs initially present, the duration of baldness, and the size of the balding area. No serious systemic or cutaneous side effects were noted.