Reorientation osteotomies are widely used for the treatment of residual dysplasia of the hip. Preoperative planning is mandatory, and 3D-evaluation is of utmost importance for precise definition of direction and magnitude of displacement of the acetabulum. Instead of a 3D-CT scan reconstruction which needs multiple slices, we developed a 3D-reconstruction from a single AP pelvic view. This method is applicable if we accept that the femoral head and acetabulum are quite spherical. Appropriate software and a PC are used for this reconstruction that we compared with 3D CT-scan reconstruction and pelvic anatomical preparations. Twenty-two patients, 17 female, 5 male, with hip dysplasia, were treated by periacetabular osteotomy according to Ganz. The mean age was 27 years (14-40 years); the mean follow-up, 4.8 years (2-7 years). Four patients were treated conservatively in infancy for DDH, one patient had snapping hips associated with hip dysplasia, and two patients had slight sequellae of cerebral palsy. All patients were symptomatic mainly during daily activities or sports. The clinical evaluation was done using the Charnley scoring system. The Charnley score for pain improved from 3.6 (2-3.5) to 5.8 (5-6) at follow-up. No restriction of mobility or of walking capacity was observed after operation. The computer-assisted method also permits appreciation of the evolution of classical coxometry; i.e. Wiberg and Lequesne angles. The improvement after Ganz osteotomy was respectively 141% and 161% for Wiberg and Lequesne angles without any posterior uncoverage. The 3D-evaluation showed an improvement of 28% of the vertical projection area of the acetabulum on the femoral head. The anterolateral coverage improved from 20.3 to 50.1%. The Ganz osteotomy is really a 3D-reorientation osteotomy. With our simplified method it is possible to predict and control the amount of displacement to be done. However, we have to keep in mind that the articular cartilage is a limited crescent in the acetabulum; this method cannot replicate exactly the form and shape of articular cartilage. At this time it allows us to better control the amount of displacement during operation to avoid too large a displacement or lateralization.