Objective: To determine the relative contribution of decline in bone density to the increase in risk of hip fracture with age in men and women. Design: Incidence data of hip fracture from the general population were combined with the bone density distribution in a sample from the same population and with a risk estimate of low bone density known from literature. Setting: The Netherlands. Subjects: All people with a hospital admission for a hip fracture in 1993, and bone density measured in a sample of 5814 men and women aged 55 years and over in a district of Rotterdam. Main outcome measure: One year cumulative risk of hip fracture by age, sex, and bone density measured at the femoral neck. Results: A quarter of all hip fractures occurred in men. Men reached the same incidence as women at five years older. Controlled for age, the risk of hip fracture by bone density was similar in men and women. The risk of hip fracture increased 13-fold from age 60 to 80; decrease in bone density associated with age contributed 1.9 (95% confidence interval 1.5 to 2.4) in women and 1.6 (1.3 to 1.8) in men. Conclusions: The risk of hip fracture by age and bone density is similar in men and women. The decrease in bone density associated with age makes a limited contribution to the exponential increase of the risk of hip fracture with age. The risk of hip fracture increases exponentially with age in both men and women Men have about the same risk of hip fracture five years later than women The risk of hip fracture by age and bone density is similar in men and women The difference in age specific incidence is explained completely by the different bone density in men and women The contribution of decline in bone density to the exponential increase in risk of hip fracture with age is relatively small