The bone mineral content (BMC) and bone mineral density (BMD) of the lumbar spine (L2-L4) and femoral neck were measured by dual-energy x-ray absorptiometry in 84 healthy Finnish children and adolescents aged 6-19 years. Both BMC (g) and BMD (g/cm2) were closely related to age, height and weight (r values from 0.724 to 0.920). When the BMD values were adjusted for age, height and weight, the mean lumbar BMD was higher in girls than in boys (P = 0.001), whereas in the femoral neck the situation was opposite (P = 0.032). Attempts were also made to normalize the BMD data for the size of bones. When BMD values were corrected for the size of bones, the correlation between age and BMDcorr (g/cm3) at the femoral neck disappeared suggesting that apparent volumetric density (g/cm3) did not change significantly during childhood and adolescence. Statistically higher femoral neck BMD and BMDcorr values were found in the study subjects, who were physically active (P less than 0.005). However, given the influence of nutrition and other environmental factors, one must be careful in interpreting the results concerning the determinants of bone mass.