Residual acetabular dysplasia leads to hip arthritis as a consequence of insufficient femoral head coverage which causes abnormal pressure on the joint cartilage. Spherical periacetabular osteotomy, described by H. Wagner, improves the acetabular coverage of the head. This surgical technique has been performed by the senior author, in Galeazzi Orthopaedic Institute of Milan, Italy since 1979. The purpose of this study is to evaluate the clinical and radiographic results of 14 patients affected by acetabular dysplasia and operated before 1985 by the same surgeon with this technique. The analysis of long term results (follow-up longer than 20 years) has shown that it is possible to achieve good clinical outcomes and to delay the need for total hip arthroplasty if a precise surgical technique is employed and if the correct indications are followed. Obese patients or hips with X-ray signs of osteoarthritis show the worst results in our experience. Our attitude is nowadays: full indication of spherical periacetabular osteotomy in dysplasia of the adolescent, indication in young adult patients only when some symptom is present, very rare indication in arthritis and only in very mild cases in young patients.