Between 1975 and 1984, we performed rotational acetabular osteotomy in 22 female patients with painful hip dysplasia. At the time of surgery, the patients were in their twenties, and radiographs showed slight narrowing of the joint space. Of these patients, 15 were followed-up for 15 to 22 years (average, 19.8 years) after surgery. The preoperative severity of coxarthrosis in all 15 hips was graded as stage II, according to the classification of coxarthrosis advocated by the Japanese Orthopaedic Association. All 15 patients available for follow-up had had no additional operations on the operated side during the follow-up period. At the time of follow-up, the patients were aged 41 to 48 years (average, 44.3 years). Of the 15 patients, 12 had little or no pain and 14 could walk for more than 30 min without a cane; the severity of coxarthrosis was graded stage I in 3 hips, stage II in 4 hips, stage III in 5 hips, and stage IV in 3 hips. We conclude that rotational acetabular osteotomy is efficacious for patients who have preoperative radiographic findings of slight narrowing of the joint space.