Spontaneous osteonecrosis of the knee is a superficial subchondral lesion classically seen in the medial femoral condyle; in general, it is markedly different in its clinicopathological presentation from the classic wedge-shaped subchondral osteonecrotic lesions seen in the hip, knee, and other joints. Recent reports on subchondral insufficiency fracture of the femoral head, which has marked morphological similarities with spontaneous osteonecrosis of the knee, led us to reevaluate a series of patients who had had operative treatment because of a clinical and pathological diagnosis of spontaneous osteonecrosis of the knee. We reviewed the cases of fourteen patients who had had operative treatment of spontaneous osteonecrosis of the knee in order to reevaluate the gross and histological morphology of this lesion. The patients included eight women and six men who ranged in age from fifty-nine to eighty-eight years. In all patients, the diagnosis of spontaneous osteonecrosis of the knee had been based on clinical presentation, imaging studies, and pathological findings. The appearance of the lesion on plain radiographs was categorized into four stages, which corresponded to the gross and histological findings. In stage 1, the radiographic appearance is normal; in stage 2, a radiolucent oval area is seen subchondrally or there is slight flattening of the convexity of the condyle, or both; in stage 3, the radiolucent area is expanded and is surrounded by a sclerotic halo; and in stage 4, secondary osteoarthritic changes are apparent. No patient had a stage-1 lesion. Three patients, all of whom had a stage-2 lesion, were considered to have a subchondral insufficiency fracture of the medial femoral condyle. Another six patients, all of whom had a stage-3 lesion, were considered to have a subchondral fracture and associated focal osteonecrosis that was confined to the area between the fracture line and the articular surface. The remaining five patients, three of whom had a stage-3 lesion and two of whom had a stage-4 lesion, had indeterminate findings because the lesion had become detached from the condyle. Our histopathological findings suggest that the primary event leading to spontaneous osteonecrosis of the knee is a subchondral insufficiency fracture and that the localized osteonecrosis seen in association with this disease is the result of a fracture.