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To explore the magnitude of common genetic and environmental effects shared by bone mineral density (BMD) and bone size (BS) in a large sample of 4,489 subjects (2,667 females and 1,822 males) from 582 Caucasian pedigrees, we performed a bivariate variance decomposition analysis to evaluate genetic correlation (ρG), environmental correlation (ρE), and phenotypic correlation (ρP) between BMD and BS at the spine and hip, as well as their “synthesized” skeletal site (bone mineral density principal component, bone size principal component) generated by principal components analysis. Significant ρG, ρE, and ρP were detected, but the shared genetic influence on BMD and BS was only 21%, 1.3%, and 11.6% at the spine, hip, and their joint variable, respectively. The results suggest that it may be important to choose both BMD and BS, especially at the hip, as surrogate phenotypes for osteoporosis genetic studies in Caucasians.