A twin study investigated the genetic and environmental contributions to bone mass in adults, focusing on the lumbar spine, proximal femur, and distal forearm. Bone mineral density (BMD) and bone mineral content (BMC) were measured in 38 monozygotic (MZ) and 27 dizygotic (DZ) twin pairs. The results showed that BMD was significantly more highly correlated in MZ twins than in DZ twins, indicating a significant genetic contribution to bone mass at these sites. However, the genetic contribution was less pronounced in the proximal femur and distal forearm, suggesting that environmental factors play a greater role in these areas. This study is the first to demonstrate a genetic contribution to bone mass in the spine and proximal femur in adults, confirming similar findings for the forearm. Bivariate analysis suggested that a single gene or set of genes determines bone mass at all sites. The study found that heritability was significantly greater than zero at all locations except the distal forearm. Analysis of premenopausal twin pairs showed that heritability was significant for the spine, forearm, and two sites in the proximal femur. The correlations between MZ and DZ twins for BMD were consistent with a significant genetic determinant of variation in bone mass. The study also found that environmental factors, such as physical activity and body weight, influence bone mass. The results support the importance of family history and suggest that DNA studies could identify individuals at risk for osteoporosis. The findings highlight the potential for lifestyle interventions to reduce the incidence of osteoporotic fractures.A twin study investigated the genetic and environmental contributions to bone mass in adults, focusing on the lumbar spine, proximal femur, and distal forearm. Bone mineral density (BMD) and bone mineral content (BMC) were measured in 38 monozygotic (MZ) and 27 dizygotic (DZ) twin pairs. The results showed that BMD was significantly more highly correlated in MZ twins than in DZ twins, indicating a significant genetic contribution to bone mass at these sites. However, the genetic contribution was less pronounced in the proximal femur and distal forearm, suggesting that environmental factors play a greater role in these areas. This study is the first to demonstrate a genetic contribution to bone mass in the spine and proximal femur in adults, confirming similar findings for the forearm. Bivariate analysis suggested that a single gene or set of genes determines bone mass at all sites. The study found that heritability was significantly greater than zero at all locations except the distal forearm. Analysis of premenopausal twin pairs showed that heritability was significant for the spine, forearm, and two sites in the proximal femur. The correlations between MZ and DZ twins for BMD were consistent with a significant genetic determinant of variation in bone mass. The study also found that environmental factors, such as physical activity and body weight, influence bone mass. The results support the importance of family history and suggest that DNA studies could identify individuals at risk for osteoporosis. The findings highlight the potential for lifestyle interventions to reduce the incidence of osteoporotic fractures.