The incidence of fractures of the distal radius in Japanese persons under 20 years of age was determined, and the bone mineral density of the radius was measured in 236 healthy Japanese children. The peak incidence of fractures occurred at 13 years of age (807 per 100,000) in boys and at 11 (300 per 100,000) in girls. Bone mineral density increased with age, but the rate of increase was not equal at the metaphysis and the diaphysis in the parapubertal period. The metaphyseal/diaphyseal ratio of bone mineral density was lowest at the age of 12-13 years in boys and 11 years in girls. The age at the peak incidence of fractures thus coincided with the age at which the metaphyseal/diaphyseal density ratio was lowest. Thus, it is suggested that low bone mineral density at the metaphysis may be the cause of the high incidence of these fractures in adolescence.