Thirty-three patients with treatment-resistant cystic and conglobate acne entered a randomized, double-blind protocol testing the efficacy of isotretinoin versus placebo. There was an overall 57% increase in the number of cystic lesions in seventeen patients who initially received placebo. Sixteen of these seventeen patients then received isotretinoin, with a resultant 98% improvement. The sixteen patients who had been randomly assigned to receive initial therapy with isotretinoin had a 95% improvement, Twenty-seven of the thirty-two patients treated with isotretinoin cleared completely. The average maximum dosage of isotretinoin received by these patients was 1.2 mg/kg/day. Eighteen patients received only one 4-month course of isotretinoin. Fifteen patients received two courses. These included twelve patients with predominantly truncal acne who responded partially to the first course, and three patients who had cleared completely after one course of therapy but had mild relapses after an average of six months off of treatment. All patients are now in remission averaging 38 months in duration. Skin biopsies and quantitative measurement of sebum production during therapy indicated a profound inhibition of sebaceous gland size and function, which may be central to the mechanism of action of isotretinoin in acne.