To study the effect of bone mass on the risk of fracture, we followed 521 Caucasian women over an average of 6.5 yr and took repeated bone mass measurements at the radius. We observed 138 nohspinal fractures in 3,388 person-yr. The person-years of follow-up and the incident fractures were crossclassified by age and bone mass. The incidence of fracture was then fitted to a log-linear model in age and bone mass. It was found that incidence of fracture increased with both increasing age and decreasing radius bone mass. When subsets of fractures were examined it was found that age was a stronger predictor of hip fractures, whereas midshaft radius bone mass was a stronger predictor of fractures at the distal forearm. We concluded that bone mass is a useful predictor of fractures but that other age-related factors associated with fractures need to be identified.