Weight is strongly associated with bone mineral density (BMD), but the mechanism of this effect is not well understood. Weight, height, hip-waist ratio, elbow breadth, adiposity, and BMD were measured in 6705 older women participating in the Study of Osteoporotic Fractures. Adiposity was measured by bioelectric impedance and BMD by single-photon (proximal and distal radius and calcaneus) and dual-energy x-ray absorptiometry (lumbar spine and proximal femur). Age-adjusted associations between weight and BMD were robust at all sites (R2 = 5.9-20.4%), but the addition of other anthropometric variables to the model only marginally improved the association. Adiposity explained a substantial fraction of the effect of weight on BMD, particularly at weight-bearing sites (36-62%). On the other hand, weight explained virtually all the variability of adiposity on BMD at weight-bearing sites (81-100%). At the radial measurement sites, adiposity had more substantial independent contributions. Weight did not seem to influence the relationship between BMD and age. In sum, at weight bearing-sites, the preponderance of the effect of weight on BMD is a direct result of mass effects rather than adiposity, whereas at non-weight-bearing sites, adiposity exerts more important effects, potentially mediated by metabolic factors.