15 patients with severe nodulo-cystic acne were treated with 13-cis retinoic acid (Ro 4-3780) for 6 to 12 months. The initial dose was 40 mg/day in all the cases. In 14 out of 15 cases this dose was progressively reduced depending on improvement of the acne lesions and on the occurrence of side effects. In one case the initial dose was later increased to 60 mg/day because of lack of response to the treatment. A 75 p. 100 improvement in the severity of the acne lesions was achieved within 4 to 5 months. Complete disappearance of the lesions was achieved in most of the patients after between 6 and 12 months of treatment. The most frequent clinical side effects were dryness of the lips, cheilitis with rhagades and facial dermatitis. Abnormal increased values of hepatic enzymes and triglycerides were found in some patients.