Background：Oral isotretinoin has been widely used in severe acne patients and is known as a very effective drug. Recently it is being used in any grades and any types of acne. However we have few reports of clinical study about oral isotretinoin in our country. Objective：To investigate clinical efficacy, factors influencing the result and side effects of oral isotretinoin in mild to moderate acne patients in Korea, prospectively. Materials and methods：The patients with mild to moderate acne visiting out-patient dermatology clinics of four university hospitals have taken oral isotretinoin 0.3-1.0mg/kg/day over 16 weeks. The clinical improvement was assessed by the scores using photographs before treatment and at 8 weeks, 12 weeks, 16 weeks, 20 weeks after treatment, and by the satisfaction of patients and physicians at the end of treatment. The side effects were recorded and SGOT, SGPT, cholesterol and triglyceride were checked at regular intervals. Results：The grade of acne was much improved proportional to the duration of treatment. The factors showing better results were dosage and duration of oral isotretinoin, older age, more severe acne and severe acne in the past. About 90% of patients and physicians were satisfied with the result at the end of treatment. The side effects, mainly mucocutaneous symptoms were expectable and tolerable in the most cases. The values of SGOT, SGPT, cholesterol and triglyceride were increased after treatment but within normal reference range. Conclusion：Oral isotretinoin is a very effective agent in Korean patients with mild to moderate acne. We expect the better result in patients having high dose and long duration of oral isotretinoin, old ages and more severe acne. If we understand it's side effects enough, we can use it very safely. (Korean J Dermatol 2000;38(10)：1309~1314)