Low bone density in women with hip fractures ("senile" osteoporosis) may be due to excessive bone loss or low peak bone density. If excessive bone loss is responsible, then no reduction in bone density is expected in their daughters. If low peak bone density is responsible, then bone density should also be reduced in their daughters because genetic and family environmental factors influence the variability in bone density. Bone density was measured using dual-photon absorptiometry and expressed as a standardized deviation or Z score relative to 697 controls, adjusting for age, height, weight, and menopausal status. In 74 women with hip fractures, the Z score (mean +/- SEM) was -0.52 +/- 0.14 (P < 0.001) at the femoral neck, -1.04 +/- 0.17 (P < 0.001) at the femoral shaft, and -0.43 +/- 0.10 (P < 0.001) at the lumbar spine. In their 41 daughters, the Z score was -0.40 +/- 0.17 (P < 0.05) at the femoral neck, -0.41 +/- 0.19 (P < 0.001) at the femoral shaft, and 0.23 +/- 0.13 (NS) at the lumbar spine. We conclude that daughters of women with hip fractures are likely to be at increased risk for hip fractures themselves because they have reduced femoral neck bone density. Femoral neck fractures may not be entirely attributable to trauma; reduced bone density is likely to contribute and may be caused by the attainment of a lower peak femoral neck bone density.