For the management of adult dysplastic hips with early degenerative arthritis, we can prevent further progress of secondary degenerative changes of the hip by a Rotational Acetabular Osteotomy, which effectively corrects the preoperatively inadequate acetabular coverage of the femoral head. Biomechanical advantages of this procedure are, l. An adequate coverage of tl; femoral head then enlarges the actual weight-bearing surface area, and so diminishes the resultant force per unit area of the acetabular weight-bearing surface and femoral head. 2. The resultant forces are also diminished by a medial shift of the femoral head. 3. The shearing force is decreased by the reduction of roof obliquity. 4. Because of the rotation of the original articular surface, remodeling the post-operative acetabular articular surface as Chiari operation is not necessary, and so can be performed even on elderly patients.