We report the incidence of fractures by sex and age based on 851 femoral shaft fractures from specific types of trauma. The maximum incidence occurred between 2 and 3 years of age, and the total incidence was 2.6 times higher in boys than in girls. In 438 cases the fractures were caused by falls and in 413 cases by traffic accidents. Fractures reported to be caused by falls were most common in children 2 and 3 years of age. Because child abuse has been shown to be involved in the majority of such fractures in early infancy, the data demonstrate the possible magnitude of this problem. Traffic accidents were most common in the oldest age groups, reaching 3.7 cases/10,000 population/year in boys 16 and 17 years of age. Although all fractures were more common during the periods of the fastest skeletal growth, the difference in incidence between different causes indicated that environmental factors are more important than endogenous factors for the risk of fracture.