Seven years after total knee arthroplasty, a patient with a well-fixed, uncemented femoral component (cobalt-chromium-molybdenum alloy) developed a large cystic lesion in the distal femur adjacent to the femoral component. This lesion contained fibrotic soft tissue, evidence of a foreign-body giant cell reaction, and a large number of polyethylene particles, but no metal wear debris, infection, or malignancy.