Understanding the determinants of adult bone mass may help to identify women for prevention of osteoporosis. We postulated that birth weight would predict low adult bone mass in old age. Subjects were 305 postmenopausal Caucasian women (mean age 70 years). Bone mineral content (BMC) and bone mineral density (BMD) were measured at the wrist, forearm, hip and lumbar spine. Birth weight was assessed by self-report. Birth weight was positively correlated with BMC at the forearm (r = 0.15), hip (r = 0.12) and lumbar spine (r = 0.18), and the age-adjusted mean BMC increased significantly from the lowest to the highest birth weight tertile. Adjusting for adult weight diminished this association at the forearm and hip, but not at the spine. Adjustment for multiple other covariates, including height, did not materially change these associations. Adult weight and height were significantly correlated with birth weight (r = 0.19 and r = 0.24, respectively). Birth weight was not independently correlated with BMD. Birth weight was thus positively correlated with adult weight and BMC 70 years later. These findings suggest that low birth weight may be a marker for future low bone mass and that different mechanisms exist for establishing the adult bone envelope (estimated by BMC) versus its density (estimated by BMD).