The conversion of the ankylosed hip to a total hip arthroplasty may be indicated if a fused hip causes low back pain, pain in the ispilateral knee, or a fibrous ankylosis is painful. Fifteen hips converted to total hip arthroplasty between Aug., 1982 and Jul., 1988 have been reviewed one to seven years after operation and the results are as follows:1. Among the 15 hips, 8 cases confirmed as fibrous ankylosis and 7 cases confirmed as bony ankylosis. 2. The causes of ankylosis is as follow tuberculous arthritis(4 cases), secondary osteoarthritis due to pyogenic hip(4 cases), rheumatoid arthritis(4 cases), post-traumatic arthritis secondary to central fracture-dislocation of hip(2 cases), and ankylosing spondylitis(1 cases). 3. The duration of immobility of the involved hip ranged from two to twenty-five years. 4. In the 14 patients, three complained of low back pain, five of ipsilateral knee pain and six of ipsilateral hip pain. 5. The lower back pain due to malposition was relieved in all cases except ankylosing spondylitis, and the pain in ipsilateral knee was also relived in all cases after hip arthroplasty. 6. The average Harris score of the fifteen hips before arthroplasty and after was 50.1 and 88.1.