Preset study was undertaken to evaluate contribution of subtrochanteric varus-derotational osteotomy(VDO) on terms of prognosis for Legg-Perthes disease, which was done if the hips are at risk during their revascularizing stage. There were four requirements for the hips to be included in this study: unilateral involvement to have opposite hips as control, treated by VDO, followup until full regeneration occurs, and complete clinical and X-ray date. Thirty-six hips met those requirements. Results of the evaluation are as follows. 1. Thirty-six hips were from 29 male and 7 females with ages at onset ranged between 4 to 6 years in 14 cases, 7 to 9 years in 19 cases and beyond 10 years 3 cases. 2. Mean follow-up period after VDO was4 year 7 months ranging between from 2 year 5 months to 9 year 7 months. 3. As for the extent of involvement according to Catterall's grouping, there were 28 cases for group III and 8 cases in group IV. 4. Clinical assessment made at last followup revealed 27 hips distributed in good, 7 hips in fair and 2 hips in poor. 5. X-ray assessment was made by migration index(MI), spherical quotient(SQ), center-edge(CE) angle and Mose's concentric circles. They were measured preoperatively and at followup, and bilaterally for comparison, MI was improved from 1.53 preoperatively to 1.28 postoperatively and SQ from 72.7% preoperatively to 92.3% postoperatively. The postoperative average CE angle was 25.1°. According to Mose's concentric circle scheme, 23 hips resulted in good, 11 hips in fair and 2 hips in poor. 6. According to clinical and radiological assessment by Sundt, 24 hips(67%) resulted in good, 8 hips(22%) in fair and 4 hips(11%) in poor. From above results, authors were able to draw conclusion that considerable percentage of the Legg-Perthes' hips at risk can be saved from poor prognosis by VDO if done as indicated.