AbstractIntroduction: Odd-skipped related 1 (Osr1) protein is a marker of intermediate mesoderm from which all definitive kidney progenitor cells develop. The loss-of-functionOSR1c.654G>A (rs12329305) polymorphism has been associated with a reduced kidney volume at birth and congenital renal anomalies.The aim of this study was to re-investigate whether theOSR1c.654A allele is associated with a smaller kidney volume (a surrogate of the congenital nephron number) in newborns, either alone or in combination withRETc.1296G>A(RETxOSR1interaction).Materials and methods: A total of 178 healthy, full-term Polish newborns were included in this study. Kidney size was measured by ultrasound on the third day of life and normalised for body surface area (TKV/BSA). TheOSR1polymorphism was identified by mini-sequencing. The reduced model without interaction and the full model including interaction terms were compared using the F test.Results: TKV/BSA did not differ across the genotype groups, t(176) = −0.45, p = 0.653 (106 ±24 and 108 ±25 mL/m2 for GG and GA, respectively). Similarly, there was no effect of the c.654G>A geno-type on TKV/BSA after controlling for the effect of covariates, F(1,175) = 0.28, p = 0.598. There was no association of theRETxOSR1interaction on the TKV/BSA.Conclusion: We did not find an association between theOSR1c.1296G> A polymorphism, alone and in combination withRETc.645G> A and kidney size in a group of healthy full-term newborns.