The present study attempted to evaluate the relative importance of familial/genetic factors in interindividual variation of plasma concentrations of vitamin D (calcidiol). Ninety-five nuclear pedigrees (187 males and 168 females, aged 18-91 and 18-86 years old respectively), from the Chuvasha Autonomy, Russia, were assessed for calcidiol and sex hormones. Age-adjusted 25-hydroxy-vitamin D (25OHD) measures showed very high pair-wise correlations between all members of families, which suggest that there may be a genetic factor transmitted within the families. To test this assumption, complex segregation analysis of non-adjusted 25OHD data was undertaken, in which genotype-sex specific dependence of the trait on age was incorporated. The analysis showed that some 44% of the 25OHD variance was attributable to all genetic and non-genetic factors incorporated into the model. About 22% of the variation was accounted for by a putative major gene effect.