Despite the obvious epidemiological significance of bone size (BS) and geometry (BG) traits as risk factors for osteoporotic fracture, very little is still known concerning the extent of their genetic determination. In the present paper we report the results of quantitative genetic analysis of a number of BG and BS indices, as well as of BMD measurements, obtained on a large pedigree-based sample (296 nuclear families, 1208 individuals) of plain hand radiographs. The families studied were all ethnically Caucasians (Chuvasha) living in small villages along the Volga River (Russia). The sample consisted of 636 men and 572 women, aged 18-91 years. To assess hand bone size we used the outcome of principal component analysis conducted on 48 measurements of metacarpal bones and proximal phalanges (PC-BS). Two BG indices, average metacarpal cortical index and breaking bending resistance index (BBRI), also measured on metacarpal and proximal phalanges were used. Again the outcome of the principal component PC-BBRI was examined in the genetic analysis. PC-BS measurements strongly correlated with body length (r = 0.75, P < 0.001) and weight (r = 0.39, P < 0.001), suggesting that they indeed reflected hand skeleton size. Familial correlations for all studied traits, adjusted for covariates (sex, age, etc.), were all highly significant statistically. For example, parent/offspring correlations ranged between 0.248 (P < 0.001) for phalangeal BMD and 0.385 (P < 0.001) for PC-BBRI. Maximum likelihood estimates of the variance component analysis confirmed these results, indicating that approximately 58 to 66% of the residual variance of the studied traits was attributable to genetic effects. Bivariate analysis clearly revealed that while genetic variation of the phalangeal BMD was independent of the genetic effects influencing hand BS and BG, the latter two were strongly interrelated. A substantial proportion of PC-BS and PC-BBRI variation was due to shared genetic (r(G) = 0.468 +/- 0.063) and environmental (r(E) = 0.704 +/- 0.052) factors.