Bone mineral density (BMD) is a measure of a person's skeletal mineral content, and assessing BMD by dual x-ray absorptiometry (DEXA) can help to diagnose diseases of low bone density. In this study, we determine the heritability of BMD in male and female monozygotic twin subjects using DEXA in 13 specific anatomical regions. In an attempt to quantify the genetic contribution of gender and skeletal region to BMD heritability, we scanned 14 pairs of identical twins using DEXA and calculated the broad-sense heritability coefficient (H(2)) in each of the 13 different body regions. The region of the body that was most heritable for both genders was the head (H(2) >or= 95%). When males were compared to females, H(2) values for male hip (H(2) = 87%) and lower extremities (H(2) = 90%) were higher than those in females (H(2) = 49% and 56%, respectively). Conversely, H(2) value for the female pelvis (H(2) = 68%) was higher than that for males (H(2) = 26%). These data show that different regions of the skeleton exhibit different degrees of heritability, and that the variation depends on gender.