A three-dimensional finite element model of a femoral head was developed using a surface modeling technique. The distribution of the stress index S (S = effective stress / yield strength, sigma/sigmaY) in various sizes of segmental osteonecrosis was assessed. The stress index of the femoral head was within physiological limits when the necrotic angle was less than 110 degrees. Within both the subchondral region and the deep necrotic region adjacent to the necrotic-viable interface, values of the stress index significantly higher than the normal physiological level (>0.1) appeared when the necrotic angle was 110 degrees or more. In the analysis of 28 osteonecrotic femoral head specimens, fracture appeared in two major locations: the deep necrotic region near the underlying necrotic-viable interface (19 femoral heads) and the subchondral region (7 femoral heads). In 2 femoral heads, the fracture involved both regions. Both sites of fracture coincided with the region of stress index greater than 0.1 in the finite element model study. These results may provide baseline information for predicting the collapse of the femoral head and determining the treatment modality of early stage osteonecrosis.