The incidence of cervical and intertrochanteric proximal femur fractures at various levels of cervical and intertrochanteric bone mineral density, respectively, was estimated by using population-based data from ongoing studies of osteoporosis and fractures among women residing in Rochester, Minnesota. Hip fractures were uncommon among women with femoral bone density ≥1.0 g/cm², but their frequency increased as bone density declined below that point at both femoral sites. The incidence of cervical femur fractures was estimated at 8.3 per 1,000 person-years among women with cervical bone density <0.6 g/cm², while the estimated incidence of intertrochanteric femur fractures reached 16.6 per 1,000 person-years among those with intertrochanteric bone density less than <0.6 g/cm². This new approach to the assessment of fracture risk from bone mineral density measurements indicates that osteoporosis is an important underlying cause of hip fractures.