Sebum production and skin surface lipid composition have been measured during the oral treatment with 13cis-retinoic acid of a group of patients with severe cystic acne. Two hundred and thirty-eight paired samples have been analyzed and compared. A marked, drug-induced decrease in sebum production was accompanied by a decrease in the concentration of wax esters, a slight decrease in the squalene concentration and an increase in the cholesterol concentration in the skin surface lipids. These changes were consonant with changes in the relative contributions of sebum and epidermal lipid to the surface film, except that at very low levels of sebum production there might be preferential preservation of squalene synthesis.