Familial resemblance in bone mineral density at five skeletal sites was measured among 160 adult members of 40 families. Each family included a postmenopausal mother, one premenopausal daughter, one son, and the children's father. Similarities in selected life-style factors thought to influence bone density, such as physical activity, smoking, alcohol use, and diet, were also evaluated. Bone density was measured by dual-energy (total body, femoral neck, and lumbar spine) or single-photon (radius and os calcis) absorptiometry. Correlation coefficients between the midparent Z score and offspring Z scores of bone mineral density ranged from 0.22 to 0.52 among daughters and from 0.27 to 0.58 among sons. Adjustment of bone density for age, height, weight, and significant life-style or environmental factors yielded heritability estimates for the five skeletal sites between 0.46 and 0.62. That is, 46–62% of variance in bone density was attributable to heredity. Most estimates derived from the group of daughters were similar to those from the sons. These observations provide support for a significant contribution of heredity to bone density. However, an individual's life-style may account for a potentially large proportion of the nonheritable variance in bone density.