The association of bone loss with age, sex, and several prevalent and modifiable potential risk factors for osteoporosis was studied in 1,856 men and 2,452 women aged 55 years and over from the Rotterdam Study, a population-based cohort study in the Netherlands. The rate of change in femoral neck bone mineral density was estimated longitudinally between 1990 and 1995, after 2 years of follow-up on average. These rates, adjusted for age and body mass index, were -0.0025 (95% confidence interval -0.0038 to -0.0012) in men and -0.0045 (95% confidence interval -0.0056 to -0.0034) g/cm/2year in women (p = 0.03). Bone loss accelerated with age, as seen more clearly in men than in women. Lower body mass index and cigarette smoking were associated with increased bone loss in both men and women. In men, higher calcium intake was associated with lower rates, and disability was associated with borderline significantly higher rates of bone loss (p = 0.07). In women, a nonsignificant relation was observed with disability, but not with dietary calcium intake. Alcohol intake was not consistently related to the rate of bone loss in either sex. It is concluded that in elderly people the rate of bone loss is higher in women, progresses with age, and is further determined by several modifiable risk factors, particularly in men. Am J Epidemiol 1998;147:871-9.