Objective To discuss the mid?to long?term results and possible prognostic factors of Bernese periacetabu?lar osteotomy for adolescents and young adults with developmental dysplasia of the hip. Methods Bernese periacetabular os?teotomy was performed on 162 patients (171 hips) with hip dysplasia from August 1997 to July 2009. Follow?up data was ac?quirable completely in 123 cases (137 hips), including 30 males (33 hips) and 93 females (104 hips). Age of surgery was 12-48 years (average 27.1 years). The X?ray radiographs and Harris scores were recorded preoperatively and at the last follow?up. The lateral center?edge angle (LCE), acetabular incline angle (AI), continuity of Shenton's line were also recorded. The grades of the hip osteoarthritis were classified by T?nnis classification. Insufficient corrections of osteotomy, excessive corrections of osteotomy, nonunion of the pubis, nerve injuries, and vascular injuries were recorded as complications. Results All the con?tactable cases was followed up again in June and July in 2014. Follow?up duration was 5-17 years (mean, 8.1 years). The LCE angle improved from 7.13° ± 8.86° preoperatively to 30.17° ± 14.98° at the final follow?up. The T?nnis acetabular incline angle decreased from 27.84° ± 12.85° preoperatively to 7.06° ± 10.58° at the final follow?up. The continuity of Shenton's line was in?tact in 106 cases postoperatively, compared with 87 cases preoperatively. Harris score improved from 83.34±9.82 preoperative?ly to 92.79±7.05 at the last follow?up. There was significant correlation between preoperative Harris score and Harris score at the last follow?up. Early complications were observed in 15 cases (15 hips), along with 17 cases (17 hips) of progressions of hip osteoarthritis. Survival rate of the hip at the last follow?up was 97.8%. Conclusion Bernese periacetabular osteotomies can significantly improve bone coverage and joint function of hip dysplasia patients with low rate of short?term complications. Further?more, there was little osteoarthritis progression observed, with good survival rate of the joint in mid?to long?term follow?up. Lower pre?operative Harris scores and higher pre?operative T?nnis classifications are important prognostic factors for post?operative pain.