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 greater than or equal to 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 less than 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.