Background: The optimal treatment of severe acetabular dysplasia with subluxation of the femoral head or the presence of a secondary acetabulum remains controversial. The purpose of this study was to analyze the extent of surgical correction and the early clinical results obtained with the Bernese periacetabular osteotomy for the treatment of severely dysplastic hips in adolescent and young adult patients. Methods: Sixteen hips in thirteen patients with an average age of 17.6 years (range, 13.0 to 31.8 years) were classified as having severe acetabular dysplasia (Group IV or V according to the Severin classification). Eight hips were classified as subluxated, and eight had a secondary acetabulum. Preoperatively, all patients had hip pain and sufficient hip joint congruency on radiographs to be considered candidates for the osteotomy. All sixteen hips underwent a Bernese periacetabular osteotomy, and six of them underwent a concomitant proximal femoral osteotomy. Postoperatively, the hips were assessed radiographically to evaluate correction of deformity, healing of the osteotomy site, and progression of osteoarthritis. Clinical results and hip function were measured with the Harris hip score at an average of 4.2 years postoperatively. Results: Comparison of preoperative and follow-up radiographs demonstrated an average improvement of 44.6° (from -20.5° to 24.1°) in the lateral center-edge angle of Wiberg, an average improvement of 51.0° (from -25.4° to 25.6°) in the anterior center-edge angle of Lequesne and de Seze, and an average improvement of 25.9° (from 37.3° to 11.4°) in acetabular roof obliquity. The hip center was translated medially an average of 10 mm (range, 0 to 31 mm). All iliac osteotomy sites healed. The average Harris hip score improved from 73.4 points preoperatively to 91.3 points at the time of the latest follow-up. Eleven of the thirteen patients (fourteen of the sixteen hips) were satisfied with the result of the surgery, and fourteen hips had a good or excellent clinical result. Major complications included loss of acetabular fixation, which required an additional surgical procedure, in one patient and overcorrection of the acetabulum and an associated ischial nonunion in another patient. Both patients had a good clinical result at the time of the latest follow-up. There were no major neurovascular injuries or intra-articular fractures. Conclusions: The periacetabular osteotomy is an effective technique for surgical correction of a severely dysplastic acetabulum in adolescents and young adults. In this series, the early clinical results were very good at an average of 4.2 years postoperatively; the two major complications did not compromise the good clinical results. Level of Evidence: Therapeutic Level IV. See Instructions to Authors for a complete description of levels of evidence.