Degenerative arthritis secondary to a dysplastic hip can appear early and is usually progressive due to its abnormal biomechanics. To prevent this secondary osteoarthritis, authors performed the rotational acetabular osteotomy for 21 symptomatic patients of acetabular dysplasia from 1984 to 1991. This method has a biomechanical advantage in treatment of dysplastic hips. It provides sufficient coverage of the femoral head by rotational displacement of the acetabulum with its intact cartilarge, the weight bearing part is enlarged by better alignment of the joint surface, and shearing forces are lessened by the decrease of the acetabular inclination and by the reduction of the resultant force applied to the femoral head as a result of its medial displacement. They were followed post-operatively for an average of 30.9 months. The mean center-edge angle improved from 7°to 43°and the angle of the roof obliquity was reduced from 25°to 2°postoperatively. Through this procedure, we were able to prevent or at least slow down further progress of secondary degenerative changes of the hip in adult dysplastic hips.