Standard projectional bone density of the femoral neck (BMD), defined as bone mineral content divided by the projected area of the neck, predicts hip fractures but may not accurately estimate the true volumetric bone density of the femoral neck. To determine whether an estimate of the volumetric bone density of the neck, "bone mineral apparent density" (BMAD), would be a better predictor of hip fracture, we analyzed dual x-ray absorptiometry scans obtained prospectively from 7963 older white women, of whom 83 suffered a hip fracture during follow-up. Both BMD and BMAD were stronger predictors than bone mineral content (BMC) of the femoral neck. However, BMD and BMAD had very similar predictive values for hip fracture: each standard deviation decrease in either BMD or BMAD of the femoral neck increased the age-adjusted risk of hip fracture 2.6- to 2.7-fold. We conclude that BMD and BMAD of the femoral neck have a similarly strong predictive value for hip fracture.(ABSTRACT TRUNCATED AT 250 WORDS)