The frequencies of the alleles of XbaI polymorphism in the apolipoprotein B gene were determined in two groups of children, 82 with high (HCG) and 86 with low (LCG) cholesterol levels. A slightly higher incidence of the X2X2 genotype in HCG was found, but the differences were not statistically significant. No relations were found between the XbaI polymorphic site and the levels of serum lipids and lipoproteins. Common XbaI polymorphism in the apolipoprotein B gene does not determine significantly the plasma cholesterol levels in childhood.