The number of nephrons is a multifactorial trait controlled by the interaction of environmental factors and genetic variants that influence the extent of branching nephrogenesis during foetal life. A correlation between renal mass and nephron number in newborns allows the use of the total kidney volume at birth as a surrogate for congenital nephron number. Since the renin-angiotensin system plays an important role in renal development we hypothesized that the common, functional insertion/deletion (I/D) polymorphism in the ACE gene might be responsible for the variation in kidney size amongst healthy individuals. We recruited 210 healthy Polish full-term newborns born to healthy women with uncomplicated pregnancies. The kidney volume was measured sonographically. Total kidney volume (TKV) was calculated as the sum of left kidney volume and right kidney volume. TKV was normalized to body surface area (TKV/BSA). The I and D alleles were identified using polymerase chain reaction. TKV/BSA in newborns carrying at least one insertion ACE allele was significantly reduced by approximately 8% as compared with homozygous newborns for the D allele (DD genotype) (105.1±23.6 vs. 114.2±28.2 cm(3)/m(2), p<0.05). The results of this study suggest that I/D ACE polymorphism may account for subtle variation in kidney size at birth, which reflects congenital nephron endowment.