Spontaneous osteonecrosis is a common cause of knee pain in older patients, but the diagnosis is often overlooked. Sudden knee pain in older women, with marked joint line tenderness and a decreased range of motion, should alert the physician to the diagnosis. Two to three weeks after the onset of symptoms, plain radiographs will usually be normal but bone scan will be markedly positive. Treatment is initially conservative. Surgical intervention (either osteotomy or arthroplasty) is reserved for patients who develop a large radiolucent lesion in the subchondral femoral condyle.