Objective To evaluate the results of rotational acetabular osteotomy (RAO) combined with debridement under arthroscope in the treatment of adult developmental dysplasia of the hip (DDH). Methods Between April 2002 and August 2007, 24 cases (29 hips) of DDH were treated with RAO combined with debridement under arthroscope. There were 2 males (2 hips) and 22 females (27 hips) with an average age of 37.7 years (range, 21-50 years). The locations were the left hip in 7 cases, the right hip in 12 cases, and both hips in 5 cases. The course of hip pain was 8-216 months (median, 30.5 months). According to Crowe DDH classification, there were 24 hips of type I and 5 hips of type II. According to Touml;nnis hip osteoarthritis classification, there were 20 hips of stage I and 9 hips of stage II. Results The mean operation time was 150 minutes (range, 120-180 minutes); the mean intraoperative blood loss was 600 mL (range, 500-700 mL); and the mean postoperative drainage volume was 200 mL(range, 50-400 mL). All incisions healed by first intention. Twenty-four cases were followed up 4.5 years on average (range, 3-8 years). At last follow-up, claudication disappeared in 16 hips and was improved in 8 hips. The Harris hip score was improved from 79.4 ± 9.8 preoperatively to 95.1 ± 8.6 postoperatively, showing significant difference (t=2.467, P=0.010). The visual analogue scale (VAS) score was improved from 5.1 ± 0.8 preoperatively to 1.1 ± 0.6 postoperatively, showing significant difference (t=2.118, P=0.011). The X-rayfilms showed union was achieved at 12-16 weeks (mean, 13.5 weeks). There were significant differences in the centre edge angle, Sharp angle, acetabular coverage rate, and acetabulum-head index between preoperation and postoperation (P lt; 0.05). Twenty hips at Touml;nnis stage I maintained after operation, among 9 hips at Touml;nnis stage II, 5 hips was improved to stage I and 4 hips maintained. Conclusion It has a satisfactory result to treat adult DDH by RAO combined with debridement under arthroscope, which may increase the congruency of hip joint, delay or prevent the progression of hip osteoarthritis.