A circumacetabular osteotomy of the acetabulum was initially done at the University of Tokyo Hospital by one of us (H. T.) in 1968. This procedure, which rotates the acetabulum, was designed to correct a dysplastic acetabulum in adolescents and adults. The surgical exposure combines both an anterior and a posterior approach. Between 1974 and 1982 this operation was performed on 103 patients (120 hips) with acetabular dysplasia, some showing early degenerative arthritis. The forty-five hips (forty-one patients) that form the basis of this report were followed for three years to eight years and ten months (average, four years and six months). Thirty hips showed only acetabular dysplasia, and fifteen were in the early stage of degenerative arthritis. The ages of the patients at the time of operation ranged from eleven to forty-two years, the majority being in the second or third decade of life. All of the forty-five hips had a preoperative center-edge angle of 10 degrees or less, but most of them had a nearly normal value after surgery. In the majority of the hips either limp or pain with exertion, or both, had disappeared, and a satisfactory range of motion had been restored.