Because clinical course of the Legg-Calve'-Perthes disease (LCPD) is varied and unpredictable, it is important that pediatric orthopedists have to know prognostic factors of LCPD in order to choose proper method of treatment. The most universally accepted prognostic factor is the patient's age at the onset of the disease, and most patient's who are less than 5 years old have been treated non-operatively. However, we believe that the extent of involvement of the femoral head is more important in this age group and that operative treatment can get good results in cases of servere head involvement. The purpose of this study is to evaluate the efficacy of femoral varus osteotomy in patients before the age of 5 years with severe involvement. We reviewed 18 patients (23 hips) who were less than 5 years old with Catterall group III (5 patients, 7 hips) or IV (13 patients, 16 hips) involvement from June 1984 to June 1994. Femoral varus osteotomies were performed in all cases. We followed up more than 2 years (range, from 24 to 130 months) and analysed clinical and radiological results. The results were as follows: 1. The mean duration from onset of the disease to the stage of repair was 16 months (range, from 8 to 27 months). 2. 2 hips (28.6%) in group III and 12 hips (75%) in group IV showed radiographic head at-risk signs. 3. Clinically all hips in group III showed good result. However, among the 16 hips in group IV, 13 hips were rated good and 3 hips were fair. Overall, 20 hips (86.9%) showed good result. 4. Radiologically all hips in group III showed good result, whereas in group IV, 10 hips were rated good and 6 hips were fair. Overall, 17 hips (78.9%) showed good result. We concluded that femoral varus osteotomy in patients less than 5 years old with severe involvement might shorten the course of disease and might be effective method.