Women with low bone density in the radius or calcaneus are at increased risk of hip fracture. To see whether bone density of the hip measured by dual X-ray absorptiometry is a better predictor of hip fracture than measurements of other bones, we assessed bone density at several sites in 8134 women aged 65 years or more. 65 women had hip fractures during a mean follow-up of 1·8 years. Each SD decrease in femoral neck bone density increased the age-adjusted risk of hip fracture 2·6 times (95% CL 1·9, 3·6). Women with bone density in the lowest quartile had an 8·5-fold greater risk of hip fracture than those in the highest quartile. Bone density of the femoral neck was a better predictor than measurements of the spine (p<0·0001), radius (p<0·002), and moderately better than the calcaneus (p=0·10). Low hip bone density is a stronger predictor of hip fracture than bone density at other sites. Efforts to prevent hip fractures should focus on women with low hip bone density.