Between 1974 and 1987, we performed 38 rotational acetabular osteotomies to treat advanced coxarthrosis caused by acetabular dysplasia in 38 patients who were aged 40 years old or less at the time of surgery. Of these patients, 28 were followed-up for more than 10 years after surgery. The preoperative severity of coxarthrosis was graded as stage III in 21 hips and as stage IV in 7 hips, according to our modification of the classification of coxarthrosis advocated by the Japanese Orthopaedic Association. At the time of follow-up, 27 patients retained their own hip joints on the operated side 10 to 18 years (average, 13 years) after surgery, and the remaining patient had had a secondary total hip replacement 7 years after the surgery. Of the 27 patients who retained their own hip joints on the operated side, 20 had little or no pain and none suffered from severe pain in the operated hip; the severity of coxarthrosis was graded as stage II in 4 hips, as stage III in 9 hips, and as stage IV in 14 hips. We conclude that rotational acetabular osteotomy can be a useful procedure in young patients who have advanced coxarthrosis secondary to acetabular dysplasia.