PURPOSE: The purpose of this study is to analyze the clinical and radiological results of performing primary total hip arthroplasty (THA) in patients with a fused hip and to compare the results with that of patients with avascular necrosis of the femur head (AVNF). MATERIALS AND METHODS: 19 patients (21 hips) with a fused hip(s) were retrospectively reviewed. We analyzed them clinically with using the Harris hip score (HHS), and radiologically with performing serial radiograms and we then compared them with the results of the patients in the AVNF group. RESULTS: The mean HHS was improved from 59.2 to 85.8. The VAS improved from 6.5 to 2.2 and the leg length discrepancy (LLD) improved from 27.5 mm to 12.5 mm. As complications, wound infection occurred in 3 hips, heterotopic ossification occurred in 8 hips and recurrent dislocation occurred in one hip. Although a radiolucent line was found in 4 acetabular cups and 6 femoral stems, there was no progression. Osteolysis was found in 1 acetabular cup. CONCLUSION: The results of THA in patients with a fused hip were excellent. Comparing with the AVNF group, improvement of the hip motion and LLD were much better. The improvement of the VAS and HHS showed similar results between the groups. However, the final results of the HHS, VAS, LLD and the range of motion of the hip were worse, and the complication rate was higher.