The results of periacetabular osteotomy, with and without intertrochanteric osteotomy, performed between January 1984 and December 1990, were reviewed for forty-two patients who had osteoarthrosis in a congenitally dysplastic hip. There were eight male and thirty-four female patients. The average age of the patients at the time of the operation was thirty-seven years (range, eleven to fifty-six years). Ten patients had combined periacetabular and intertrochanteric osteotomies. The preoperative severity of the osteoarthrosis, according to the criteria of Tönnis, was grade 1 in fifteen patients, grade 2 in eighteen, and grade 3 in nine. The average duration of follow-up was four years (range, two to eight years). Complications included heterotopic ossification in fourteen patients (33 per cent), non-union of the pubic osteotomy in two patients (5 per cent), and pain related to the hardware that led to its removal in nine patients (21 per cent). There were no major neurovascular complications. One patient had dysesthesias in the area supplied by the lateral femoral cutaneous nerve. Six patients had a subsequent total hip arthroplasty and three patients had an additional intertrochanteric osteotomy. five of the nine patients who had a second major operation had had grade-3 osteoarthrosis before the periacetabular osteotomy. The Harris hip score improved from an average of 62 points (range, 33 to 95 points) preoperatively to an average of 86 points (range, 29 to 100 points) postoperatively (p < 0.0001, paired t test). The result was excellent or good for thirty-two of the thirty-three patients who had had grade-1 or 2 osteoarthrosis. Eight of the nine patients who had had grade-3 osteoarthrosis had a Harris hip score of less than 70 points at the latest follow-up evaluation.