It is important to recognize idiopathic osteonecrosis or avascular necrosis of the patella not only because of its rarity but because a majority of the patients remain asymptomatic without treatment. The etiology of osteonecrosis is unclear. Certain conditions and diseases have been associated with osteonecrosis of the patella including the systemic administration of steroids, a fracture of the patella, a total knee arthroplasty and other traumatic conditions. The diagnosis is usually made with radiographs, MR imaging and a radioisotopic bone scan. It is believed to have a benign course and a favorable prognosis. We report for the first time in Korea a patient with idiopathic osteonecrosis diffusely involving the whole patella without any obvious cause.