A 77-year-old woman with severe varus and flexion deformity of both knee joints, hypertension, and arteriosclerosis underwent bilateral total knee arthroplasty. Six weeks after the operation on the right knee and four months after the operation on the left knee, osteolysis of the patellas was seen on the radiographs. Three years after the operations, the lateral and inferior poles of both patellas were sclerotic and displaced. There was no knee pain and no extension lag throughout the follow-up period. This case suggests that the prognosis of patellar osteonecrosis after total knee arthroplasty is good if the continuity of the retinaculum and the extensor mechanism are preserved, and if instability of the knee does not occur.