The possible role of genetic and/or environmental factors in determining bone mass has been investigated in 30 pairs of twins (16 monozygotic and 14 dizygotic) divided in two age groups (below and above 25 years of age). Bone mineral content was evaluated by single- and dual photon absorptiometry at the distol third of the radius for peripheral cortical bone and in the lumbar spine for the axial bone. The "within pair" variance has been used as an index of genetic influence. A significant (p less than 0.01) genetic determinant was found for the bone mass of the radius in adults and for the spinal bone mass in the age group younger than 25 years. The heritability index h2 was 0.75 for cortical BMC and 0.88 for axial BMC. Such a genetic determinant could not conclusively be demonstrated in adult twins for the spine and in youngsters for the cortical bone, suggesting that environmental factors may play a more dominant role in growth of cortical bone during adolescence and diminution of axial bone during adult life.