A prospective study was carried out on fifty hips in forty-eight patients with Ficat stage III (Steinberg stage IV) avascular necrosis (head deformity evident on plain x-ray) involving the supero-ventral segment. Mean age was 33 +/- 8 years (mean and standard deviation). There were thirty-six male and twelve female patients. Followup was 3-14 years (mean 8 years). This study precluded patients who were more than 45 years old, had an underlying condition requiring continued chemotherapy or cortisone, who had extensive dorsal (posterior) head segment involvement or those who were poorly motivated. Avascular necrosis was associated with alcohol in 35% of hips, trauma in 26% and 39% were idiopathic. One pregnancy related case was included in the latter subgroup. Curettage of the avascular segment and impaction of the bone graft was performed via subcapital fenestration of the anterior femoral neck. Pain was the presenting symptom in all cases. The mean pre-operative Harris hip score was 36 points and the mean score at last assessment was 87 points. Seven hips (14%) failed, failure implied conversion to a hip replacement or a Harris hip score of less than 70 points. Failures were seen within the first 3 years after osteotomy. Kaplan Meyer Survivorship analysis demonstrated an overall survivorship probability of 86% at 14 years. One patient died 10 years post surgery, the head was retrieved for histological study. Statistical study (Mann-Whitney U test) was carried out to determine factors related to failure. Of significance were age of the patient at time of osteotomy, contralateral hip involvement and size of the lesion. X ray studies of the femoral head included measurement of the necrotic angle, evaluation of restoration and maintenance of head contour and graft incorporation or stabilisation. Subsequent CAT scan studies enabled accurate quantification and siting of the avascular segment. Secondary x ray degenerative changes of the joint tended to manifest with time but these changes had little bearing on the clinical outcome. Osteotomy does have a measure of unpredictability, but in the appropriate case a high incidence of satisfactory results would be anticipated.