Peak bone mass is determined mainly by genetic-ethnic factors, but environmental factors such as calcium intake and physical activity during childhood and adolescence could play a role. We have measured the bone mineral density (BMD) of 151 healthy children and adolescents, ages 7-15.3 years. Density was measured by dual X-ray absorptiometry (DXA) at two sites (lumbar verterbrae L1-L4 and the upper femur), and the data were analyzed in terms of the height, weight, sexual maturation, spontaneous calcium intake, and physical activity. Of the children, 57-71% had calcium intakes below 1000 mg/day. BMD increased with pubertal maturation from 0.68 +/- 0.08 to 0.92 +/- 0.09 g/cm2 (vertebral bone density, VBD) and from 0.87 +/- 0.10 to 1.03 +/- 0.09 g/cm2 (femoral bone density, FBD) between Tanner stage 1 and 5. Multiple regression analysis showed that body weight and Tanner stage were main determinants of bone density when expressed as g/cm2. The weekly duration of sports activity also influenced both the vertebral (p < 0.001) and femoral (p = 0.01) sites, especially in girls and during puberty. Dietary calcium appeared to be another independent determinant of BMD, especially before puberty, at the vertebral (p = 0.02) site. Most important, dietary calcium was found to be the main determinant of vertebral mineral density, when expressed as z score, in both sexes. Moreover, 93% of the 28 children with low vertebral z score values (below -1) and 84% of the 31 children with low femoral z score values (below -1) had dietary calcium intakes below 1000 mg/day.