Involutional osteoporosis is a risk factor for femoral neck fracture in elderly people. This study was designed to evaluate the bone mass and quality of the femoral neck specimen obtained directly from the femoral neck at the time of hip surgery. An iliac bone biopsy was also performed in some cases. A histomorphometric analysis of the femoral neck was performed in 31 women with a femoral neck fracture, aged 64-104 (mean age 76.2), and in 19 women with osteoarthritis of the hip, aged 50-77 (mean age 64.3) as a control. The cortical thickness of the lateral, anterior and posterior aspects of the femoral neck was significantly less in the fracture group than in the osteoarthritis group. However, there was no significant difference in the medial cortical thickness between the two groups. The fracture group exhibited an increased resorption cavity area and intracortical porosity in the cortical area as well as increases in the eroded surface and osteoclast number in the endocortical envelope. The bone volume in the cancellous envelope of the medial femoral neck was significantly lower in the fracture group. Most parameters of the iliac bone were not correlated with those of the femoral neck in either group. Our findings suggested that a femoral neck fracture was associated with a decreased cortical thickness in the femoral neck. Increased intracortical porosity and a low cancellous bone volume were also critical factors in femoral neck fragility, and an iliac bone biopsy did not necessarily reflect the femoral neck bone status.