The conversion of the fused hip to THA may be used for patients with painful pseudoarthrosis, and malposition of the arthrodesed hip. It may also be used for patients with disabling pain in the ipsilateral knee, contralateral hip or lower back. Nineteen fused hips have been converted to total hip arthroplasty from January 1985 to December 1993. They were followed up for an average of 6 years. The causes of the fused hip were post-traumatic osteoarthritis (8 cases), tuberculous arthritis (6 cases), ankylosing spondylitis (3 cases) and rheumatoid arthritis (2 cases). The results of THA were satisfactory in all cases. The average Harris hip score after THA was improved from 56 to 84.1. The results of THA were best in patients with tuberculous arthritis and poorest in patients with ankylosing spondylitis. The average range of motion after THA is as follows: flexion 82 degree, abduction 28 degree, adduction 16 degree, external rotation 27 degree and internal rotation 13 degree. The average leg length discrepancy improved from 3 cm to 1.5 cm. The conversion of the fused hip to THA improves the patient s quality of life, so it is a useful treatment for the fused hip.