The bone mineral density (BMD, g/cm2) of the lumbar spine (L2-L4) and femoral neck was measured twice with a 1-year interval by dual energy X-ray absorptiometry (DEXA) in 65 healthy children and adolescents aged 7-20 years. In addition, the BMD values were corrected for the size of bones to obtain the bone volumetric density (BMDvol, g/cm3) using a method developed previously. The annual increases of BMD and BMDvol in both spine and femoral neck were most marked in females at the time of menarche (during the age of 11-13 years), and in males between the ages of 13 and 17 years. The males showed significantly higher values in their mean annual increment rates of femoral bone mineral content (BMC) and femoral neck width, whereas no differences in spinal parameters were found. The acquisition of bone mass and bone density stopped or markedly diminished before the age of 20 years, supporting the theory that the major portion of the peak bone mass is attained in late adolescence. We could not find any significant relationship between the increment rate of bone density, and physical activity or calcium intake. This study emphasizes the significant effect of puberty and genetic factors on the development of bone mass and density.