Introduction. Transtrochanteric curved varus osteotomy is one of the effective joint-preserving operations for osteonecrosis (ON) of the femoral head. We correlated various factors with the radiological outcome of this procedure. Methods. We reviewed 74 hips in 63 patients who had a minimum follow-up of 5 years after transtrochanteric curved varus osteotomy for the treatment of ON. There were 28 men and 35 women who had a mean age of 33 years (range, 15 to 68 years) at the time of surgery. Clinical assessment was made based on the Harris hip score (HHS). Radiographically, we investigated various factors; affected lesion, stage and type (localization of the necrotic lesion) of ON, varus degree, post-operative intact ratio, progression of collapse, and joint-space narrowing. On the basis of postoperative radiographs, the hips were divided into 2 groups (Group I: either the progression of collapse or joint-space narrowing, Group II: neither progression of collapse or joint-space narrowing). The related factors with radiological outcome were analyzed by using multivariate analysis (Stepwise discriminant analysis). Results. Sixty-seven of 74 hips (90.5%) survived without conversion to total hip arthroplasty at final follow-up. The mean preoperative HHS was 60 points (range, 34 to 85 points), which improved to 85 points (range, 51 to 100 points) at final follow-up. Thirteen hips were categorized as Group I and 61 hips as Group II. Both the post-operative intact ratio and type of ON affected the radiological outcome. Regarding post-operative intact ratio, the cutoff point to prevent the progression of collapse was 32.1%, and to prevent both the progression of collapse and joint-space narrowing was 42.6%. Conclusion. The results of this study indicated that the post-operative intact ratio should be at least 32.1%. This critical ratio would be useful for surgical planning in transtrochanteric curved varus osteotomy for ON