Lumbar spine bone mineral density in a cross sectional study of healthy subjects increased by 0.012 and 0.016 g/cm2/year in boys and girls respectively between 5 and 11 years of age. These rates increased five-fold in girls and threefold in boys between the ages of 11 and 13 years as a result of the bone mineral content increasing more rapidly than the coronal area at this age. By the age of 11 years the girls had 66% of the coronal area, 61% of the bone mineral density, and 41% of the bone mineral content of subjects aged 18-23 years. The ratio (lumbar spine bone mineral content/body weight) was constant in boys aged 6-13 years, but there were significant variations in girls. Femoral neck bone mineral density in both sexes changed little between 6 and 11 years and at 11 years was 69% of the adult values. Subjects with osteogenesis imperfecta had a low bone mineral density and bone mineral content for their age and weight. The z score of bone mineral density at the femoral neck was significantly lower than at the lumbar spine. In patients with recurrent fractures a low bone mineral density may help in identifying those with osteogenesis imperfecta and assist in their subsequent management.