Hip fractures are the most serious complication of osteoporosis. Although low proximal femoral bone mineral density (BMD) does not cause hip fractures directly, it is clearly a prerequisite for the increased risk associated with aging. To investigate the mechanism of the age-related decline in proximal femoral bone mineral density, we have examined the relative importance of muscle strength, physical fitness, and body mass index (BMI) in addition to age in the determination of proximal femoral BMD in 73 healthy female volunteers age 20-75 years. Muscle strength was an independent predictor of BMD at all three sites in the proximal femur as well as in the lumbar spine and forearm; proximal femur BMD was also predicted by physical fitness. BMI was a positive predictor of bone mass at all sites. In the proximal femur, age was not an independent predictor of BMD at any site. In postmenopausal women muscle strength was a significant predictor of bone mass in the femur and forearm, but not in the spine. However, BMI remained predictive of bone mineral at all sites. Muscle strength, physical fitness, and weight appear to exert independent effects upon bone mass. Age effects may be mediated indirectly through associated changes in these factors. The integrated physical load on the skeleton may be a final common pathway.