Six patients with osteonecrosis of the medial femoral condyle were seen during a 3 1/2-year period in a rheumatology unit. This condition, which may not be rare, is characterized by the sudden onset of pain in the knee of elderly people (usually women) without significant trauma. Physical signs include effusion and tenderness at the medial margin of the knee joint. Radiographic changes may not be evident for weeks or even months. The later appearances are characteristic and the site of the lesion is remarkably constant.