The aim of our study was to validate a hip fracture risk function, composed of age and femoral neck bone mineral density (BMD). This estimate of the 1-year cumulative risk was previously developed on the basis of Dutch hip fracture incidence data and BMD in men and women. A cohort of 7046 persons (2778 men) aged 55 years and over was followed for an average of 3.8 years. The 1-year hip fracture risk estimate was calculated for each participant according to the risk function and categorized as low (<0.1%), moderate (0.1 to < 1%), or high (> or =1%). Observed first hip fracture incidence was then analyzed for each of these risk categories by age and gender. Additionally, we calculated the relative risk per standard deviation (SD) decrease in femoral neck BMD in this population. At baseline, 2360 individuals were categorized as low risk, 2567 as moderate risk, and 378 as high risk During follow-up, 110 first hip fractures were observed corresponding to an incidence rate of 4.1/1000 person-years (pyrs) (95% confidence interval 3.4-5.0). The observed incidence rate in the low risk group was 0.2/1000 pyrs (0.1-0.9), 2.7/1000 pyrs (1.8-3.9) in the moderate risk group, and 18.4/1000 pyrs (12.4-27.2) in the high risk group. Below the age of 70 years, incidence was low in all categories, and very few individuals were considered at high risk Above the age of 70 years, the observed incidence was high in the high risk group, while in the low and moderate risk groups, the incidence remained low even over 80 years of age. In women, the age-adjusted relative risk for hip fractures was 2.5 per SD decrease in femoral neck BMD (1.8-3.6), while in men this relative risk was 3.0 per SD (1.7-5.4). In conclusion, we observed a similar relation of hip fracture with femoral neck BMD in men and women and were able to predict accurately hip fracture rates over a period of almost 4 years.