The risk of hip fracture is commonly expressed as a relative risk. The aim of this study was to examine the utility of relative risks of hip fracture in men and women using World Health Organization (WHO) diagnostic criteria for low bone mass and osteoporosis. Reference data for bone mineral density (BMD) at the femoral neck, from the third National Health and Nutrition Examination Survey (NHANES III), were applied to the population of Sweden. Relative risks (RRs) were calculated from the known relationship between BMD at the femoral neck and hip fracture risk. The apparent prevalence of low bone mass and osteoporosis depended on the segment of the young population chosen for reference ranges. Using a reference derived from women aged 20-29 years, the prevalence of osteoporosis was 21.2% in women between the ages of 50 and 84 years and 6.3% in men. The RRs associated with osteoporosis depended markedly on the risk comparison. For example, in men or women aged 50 years, the RR of hip fracture in those with osteoporosis compared to those without osteoporosis was 7.4 and 6.1, respectively. The RR of those at the threshold value for osteoporosis compared to those with an average value for BMD at that age was 6.6 and 4.6 in men and women, respectively. RRs were lower comparing those at the threshold value compared to the risk of the general population at that age (4.2 and 2.9, respectively). When RR was expressed in relation to the population risk rather than to the risk at the average value for BMD, RR decreased at all ages by 37%. Such adjustments are required for risk assessment in individuals and for the combined use of different risk factors. Because the average T score at each age decreased with age, the RR of hip fracture at any age decreased with advancing age in the presence of osteoporosis. The decrease in relative risk with age is, however, associated with an increase in absolute risk. Thus, for clinical use, the expression of absolute risks may be preferred to relative risks.