Total hip replacement arthroplasties(THRA) have been done in 19 patients(31 hips), who were diagnosed ad ankylosing spondylitis from January, 1984 till July, 1993 at Seoul National University Hospital. All patients were male; 12 patients were affected bilaterally. Modified New York criteria was used as diagnostic criteria. HLA B27 antigen was positive in 18 cases(95%). The thoracolumbar spine and sacroiliac joint were affected in all cases and cervical spine was affected in five patients, among them, fiberoptic laryngoscopic intubation was needed in four cases. The trastrochanteric approach was used in 19 cases, which have severe limitation of motion and deformity of the acetabulum. For the prevention of postoperative heterotopic ossification, low dose radiation therapies were done in 10 cases of the risk group. The average follow-up period was three year and eleven months(two year-seven year and 10 months). Harris hip score and radiographs were obtained at each follow-up. The mean preoperative Harris hip score was 50(19–90), and finally, the mean Harris hip score was 87(68–96). Loosening of endoprothesis was noted in 3 cases and heterotopic ossification in 5 hips. Ischial enthesopathy was observed in 13 cases. Among the hips which had heterotopic ossification, four were class ? by Brooker's classification and only one was class ?? No nonunion was observed in the cases which had trochanteric osteotomy expect breakage of wire in 4 cases. In THRA of ankylosing spendylitis, we obtained good result clinically and radiologically. In severely affected cases, we had used transtrochanteric approach and obtained good result due to better operative field. The incidence of heterotopic ossification was not significant and the prophylactic low dose radiation therapy seems to be effective in risk group.