From September 1982 to June 1987, all members (N = 513) of the Group Health Cooperative of Puget Sound, Seattle, Wash, who were prescribed isotretinoin for acne were observed throughout the first 4- to 5-month course of therapy for effectiveness and adverse effects. The highest rates of use were among male subjects aged 15 to 24 years. Excluding 47 subjects whose prescriptions were stopped because of noncompliance or who left the care of Group Health Cooperative physicians, 39 (8.4%) of the remaining 466 discontinued taking the drug because of the following adverse effects: mucous/skin/musculoskeletal effects (17); elevated triglyceride levels (eight); headaches (five); increased liver enzyme levels (three); amenorrhea (two); and other (four). One subject, excluded from the 466 because of noncompliance, became pregnant while using medication from a previous prescription and had a therapeutic abortion; she was not under the care of a physician at the time of pregnancy. Most subjects (97%) developed a mucocutaneous symptom, and 42% developed musculoskeletal symptoms. Moderate elevations in liver enzyme levels developed in six (1.8%) of 341 subjects with normal baseline values. Of 389 subjects with normal baseline triglyceride values (less than 2.25 mmol/L), nine (2.3%) developed moderate elevations (4.5 to 9.0 mmol/L), and one (0.3%) developed a severe elevation (greater than or equal to 9.0 mmol/L). Of 24 subjects with elevated baseline triglyceride levels, three (12.5%) developed moderate elevations. Of an additional 53 subjects whose baseline serum triglyceride levels were not determined, two developed elevations during therapy, one up to 13.4 mmol/L. Subjects who were overweight or had elevated baseline serum triglyceride values had an increased risk of developing elevations in triglyceride levels during therapy (odds ratio, 6.0; 95% confidence interval, 1.6 to 22.0; and odds ratio, 4.35; 95% confidence interval, 0.9 to 20.2, respectively). Acne was improved for at least 94.0% of subjects.