A total of 188 acne patients (113 male and 75 female) with a mean age of 25 years (range 15–42 years) were treated with isotretinoin in doses ranging from 0.5 to 1 mg/kg. The study lasted 9 years. The treatment was not terminated until 2 months after total healing. The patients were then re examined at regular intervals. In the event of a recurrence greater than grade 2 (pre-nodular threshold grade), they underwent a new course of treatment. At the end of the study, three groups were distinguished: (1) immediate, long-term, stable remissions following one single course of isotretinoin (111 patients with an average follow-up period of 27 months); (2) stable remissions following 2 or 3 courses of isotretinoin (54 patients with an average follow-up period of 16 months); (3) immediate partial remissions or partial remissions following several courses of treatment in 23 patients who continued to present with at least grade 3 acne. There was no significant statistical difference between the first two groups with respect to the age and sex of the patients and the grade and prior duration of the acne. The third group differed from the other two in having a greater proportion of patients with microcystic acne (p < 0.05) and women with endocrinological problems ((p < 0.001). Analysis of these results confirmed the importance of the total cumulative dose (in one or more courses of treatment) for obtaining stable remission, the existence of a threshold dose of about 150 mg/kg which, in many cases, should not be exceeded, and the need to take into account the type of acne and the hormonal status of female patients before beginning treatment with isotretinoin.