Osteonecrosis of the patella, although uncommon, has become important to recognize because it can be a cause of pain in the knee. We describe the imaging manifestations of nontraumatic osteonecrosis of the patella in seven clinical cases. The lesions uniformly involved the superior aspect of the patella. Conventional radiography displayed increased radiodensity, subchondral radiolucent areas, and typical demarcation line surrounding the ischemic region. MR imaging and bone scintigraphy demonstrated the characteristic features of osteonecrosis. Recognition of the imaging findings of osteonecrosis involving the patella can preclude misdiagnosis and may obviate unproductive invasive diagnostic procedures.