The relative importance of osteoporosis and risk of falling in causing hip fracture is not known. Femoral neck bone mass was measured in 708 elderly people, all of whom had fallen and injured a hip. Below 75 years of age there was a steep increase in relative risk of fracture with reduced bone mass, measured by Singh grade. Above that age the increase in risk was small, and neuromuscular responses which protect the skeleton against trauma may be more important than bone mass.