We evaluated the results of polygonal triple (Kotz) osteotomy for the treatment of acetabular dysplasia over 10 years. This study included 31 hips of 27 patients who had the Kotz osteotomy for acetabular dysplasia. The mean age was 21.5 years. We performed the original Kotz osteotomy for the first 22 hips ( group I), while the modified Kotz osteotomy through an intra-pelvic approach without damage to the abductor muscle was applied for the last 9 hips ( group II). Patients were evaluated by clinically and radiologically. The average follow-up was 106 months in group I, and 18 months in group II. The Trendelenburg gait was unchanged for four patients in group I and for one patient in group II. The Harris Hip Score improved in all patients postoperatively. Radiographic assesment showed improvement in both groups in terms of the angle of CE, VCE, and Sharp postoperatively(P<0.05). The complication rate per hip was 0.29. The original Kotz osteotomy achieves adequate coverage for the treatment of acetabular dysplasia, and patients are generally satisfied by this procedure. Nonetheless, the modified Kotz osteotomy provides recovery of the abductor muscle strength in the early postoperative period and subsequently decreases the rate of the Trendelenburg gait compared to the original Kotz osteotomy.