Total hip arthroplasty for ankylosed hip has a higher risk of failure due to the poor condition of the hip abductors and the altered configuration of the bone, which lead to poor positioning or fixation of the prosthesis. Fifty-one hips (42 patients) converted to total hip arthroplasty between March 1983 and March 1994 have been reviewed three to thirteen years six months after operation. The average period of follow-up was four years nine months. Twenty six of the patients were men and sixteen were women. The average age at the time of operation was 40 years old (range,19 to 70 years). Among the 51 hips, bony ankylosis were 27 hips and fibrous ankylosis were 24 hips, spontaneous ankylosis were 40 hips and surgical ankylosis were 11 hips. The duration of hip fusion was 13 years six months in the bony ankylosis and 16 years one month in the fibrous ankylosis. Preoperatively 33 cases had low back pain, ipsilateral or contralateral knee or hip pain. In asymptomatic 18 cases, conversion total hip arthroplasty was also performed for the management of functional disabilities due to severe malpositioned ankylosis. At the time of the last follow-up, 25 cases were free from pain and six patients had a mild degree pain on the hip, knee or thigh. In bony ankylosis, the postoperative average hip motion ranged from 1.1 degrees to 93.2 degrees (average arc of motion of 92.1 degrees). In fibrous ankylosis, the postoperative average hip motion ranged from 1.7 degrees to 96.6 degrees (average arc of motion of 94.9 degrees). The average Harris hip score was improved from 66 to 86.9 in bony ankylosis and from 49 to 89.5 in fibrous anklyosis. The postoperative leg length discrepancy was 1.1 cm in average. The postoperative complications included heterotropic ossification in 25 cases (49%), cup loosening in two cases (3.9%) and stem loosening in three cases (5.6%). As for the clinical results, 24 cases (89%) of bony ankylosis and 23 cases (96%) of fibrous ankylosis had satisfactory functional results at the last follow-up. In conclusion, the conversion total hip arthroplasty could provide functional improvement, pain relief and especially correction of leg length discrepancy in most cases of ankylosed hip.