Between 1970 and 1979, 17 patients underwent conversion of ankylosed hip to total hip arthroplasty. The patients' ages ranged from 38 to 82 years and the duration of fusion prior to conversion ranged from five to 60 years. The primary indication for conversion to total hip replacement in all patients was disabling pain in the low back, the ipsilateral knee, and the contralateral hip. The reason for ankylosis varied: five patients had previously undergone fusion for posttraumatic arthritis, two patients for tuberculous arthritis, six patients for osteoarthritis, one for a previous old slipped capital femoral epiphysis which had undergone spontaneous fusion after internal fixation, and two patients for ankylosing spondylitis. Relief of preoperative pain in the lower back was seen in 12 of 13 patients, knee pain was relieved in four out of four patients, and contralateral hip pain was relieved in seven out of 10 patients. Complications included heterotopic ossification, infection, perforation of the posterior shaft of the femur, and failure of trochanteric fixation with subsequent dislocation. Predictable pain relief from pain in the contralateral hip was less predictable and directly related to the degree of osteoarthritis in the contralateral hip. In these patients it is suggested that conversion of the fused hip to total hip arthroplasty be staged with total hip replacement in the contralateral hip as well. The end results were least satisfactory in patients with ankylosing spondylitis.