A multicenter study of the effectiveness of 13-cis-retinoic acid (isotretinoin) in lamellar ichthyosis and epidermolytic hyperkeratosis has been conducted. A dose of the drug which produced maximum clearing with minimum side effects was chosen; this varied among different patients, the mean dose being about 2 mg/kg/day. Almost all of the patients in both groups were clearly improved, as evaluated both by the physicians and the patients. The degree of improvement seemed higher in the group of patients with lamellar ichthyosis.