The 2001 revised criteria for the diagnosis, classification, and staging of idiopathic osteonecrosis of the femoral head were proposed in June 2001, by the working group of the Specific Disease Investigation Committee under the auspices of the Japanese Ministry of Health, Labor and Welfare, to establish criteria for diagnosis and management of idiopathic osteonecrosis of the femoral head. Five criteria that showed high specificity were selected for diagnosis: collapse of the femoral head (including crescent sign) without joint-space narrowing or acetabular abnormality on x-ray images; demarcating sclerosis in the femoral head without joint-space narrowing or acetabular abnormality; "cold in hot" on bone scans; low-intensity band on T1-weighted MRI (bandlike pattern); and trabecular and marrow necrosis on histology. Idiopathic osteonecrosis of the femoral head is diagnosed if the patient fulfills two of these five criteria and does not have bone tumors or dysplasias. Necrotic lesions are classified into four types, based on their location on T1-weighted images or x-ray images. Type A lesions occupy the medial one-third or less of the weight-bearing portion. Type B lesions occupy the medial two-thirds or less of the weight-bearing portion. Type C1 lesions occupy more than the medial two-thirds of the weight-bearing portion but do not extend laterally to the acetabular edge. Type C2 lesions occupy more than the medial two-thirds of the weight-bearing portion and extend laterally to the acetabular edge. Staging is based on anteroposterior and lateral views of the femoral head on x-ray images. Stage 1 is defined as the period when there are no specific findings of osteonecrosis on x-ray images, although specific findings are observed on MRI, bone scintigram, or histology. Stage 2 is the period when demarcating sclerosis is observed without collapse of the femoral head. Stage 3 is the period when collapse of the femoral head, including crescent sign, is observed without joint-space narrowing. Mild osteophyte formation in the femoral head or acetabulum may be observed in stage 3. Stage 3 is divided into two substages. In stage 3A, collapse of the femoral head is less than 3 mm. In stage 3B, collapse of the femoral head is 3 mm or greater. Stage 4 is the period when osteoarthritic changes are observed.