Abnormalities in plasma lipids are a recognized side effect of isotretinoin therapy for nodulocystic acne. We studied 60 patients during 20 weeks of isotretinoin therapy, to measure changes in plasma lipids and lipoproteins, to compare plasma lipid responses in men and women, and to determine whether there are alterations in levels of lipoprotein lipase or hepatic triglyceride lipase that could explain the development of isotretinoin-induced hypertriglyceridemia. Mean triglyceride levels rose in men and women, with maximum mean increases of 46.3 mg per deciliter (P less than 0.0001) and 52.3 mg per deciliter (P less than 0.002), respectively. The maximum level was reached by 4 weeks of therapy in men but not until the 12th week in women. Nine of 53 patients (17 per cent) completing 20 weeks of isotretinoin therapy acquired hypertriglyceridemia, with values of 200 to 600 mg per deciliter. Both men and women had significant increases in mean plasma levels of cholesterol and low-density-lipoprotein cholesterol and decreases in mean levels of high-density-lipoprotein cholesterol. There were no significant changes in mean levels of lipoprotein lipase or hepatic triglyceride lipase. Plasma lipid and lipoprotein levels returned to base line by eight weeks after discontinuation of the drug. If sustained over a long period, the change in the ratio of low-density-lipoprotein cholesterol to high-density-lipoprotein cholesterol that we observed, from 2.4 to 3.0 (P less than 0.0001), would predict an increased risk of cardiovascular disease.