This study determined whether patients with severe knee disease below a hip arthrodesis can be treated successfully with total knee replacement alone or whether such patients require total hip arthroplasty followed by knee replacement. Eighteen patients who had hip arthrodesis for a mean of 33 years underwent total hip replacement alone, total knee replacement alone, or a combination of both. The Harris hip score improved from a mean of 55.3 to a mean of 86.9 points at 45 months after total hip arthroplasty. The Hospital for Special Surgery knee score improved from a mean of 33 to a mean of 78 points in patients who had total knee replacement after total hip arthroplasty. The Hospital for Special Surgery knee score improved from a mean of 35 to a mean of 44 points in patients having total knee replacement alone below a hip arthrodesis. The followup after total knee replacement averaged 53 months. These data suggest that a knee replacement alone in a patient with a fused hip is unlikely to provide a satisfactory result. Patients with severe knee disease below hip arthrodesis require total hip arthroplasty followed by knee replacement. This applies even when severe osteoarthritis of the knee is the primary complaint.