Between 1973 and 1984, 70 modified Dega acetabuloplasties done simultaneously with intertrochanteric osteotomies in 51 patients with developmental dysplasia of the hip were reviewed for long-term results and analyzed for causes of failure. Mean patient age at operation was 2.9 years (range, 8 months-8 years), and mean followup time was 15.2 years (range, 10-19 years). Topical investigations were based on clinical criteria of the Severin classification and radiologic criteria of the Commission for the Study of Hip Dysplasia of the German Society of Orthopaedics and Traumatology. The review of clinical documents and investigations very good and good results in 80% of the hips. The Trendelenburg sign was a useful clinical indicator for problematic cases. Measurements on radiographs were classified into deviation grades from the normal range. Values of the acetabulum and the acetabulum to head relation were normal or slightly abnormal in >80% of hips. Measurements of the femoral head and neck were in the low normal range. The lowest percentage of normal values was in 24 cases with coxa valga after acetabuloplasty combined with derotational varus osteotomy. Poor clinical and radiologic outcome usually was the result of avascular necrosis of the femoral head or recurring valgus deformity after derotational varus osteotomy. The rate of avascular necrosis as a consequence of the operation was 5.7%. The risk of avascular necrosis caused by derotational varus osteotomy is lower than the risk of later development of deformities in the proximal femur.