To evaluate whether variations in the ABCB1, ABCC2, SLCO1B1, CYP3A4, CYP3A5, or NR1I2 genes are associated with the pharmacokinetics of cyclosporine in pediatric renal transplant candidates, and whether the effects of these variants are related to age.A total of 104 pediatric patients (aged 0.36-16.3 years) were genotyped for 17 putatively functionally significant sequence variations in the ABCB1, SLCO1B1, ABCC2, CYP3A4, CYP3A5, and NR1I2 genes. The patients had undergone a pharmacokinetic study with intravenous and oral ciclosporine (INN, cyclosporin) before renal transplantation.In the whole population, the mean+/-SD cyclosporine oral bioavailability was 0.38+/-0.09, volume of distribution was 2.3+/-0.54 l/kg, and systemic clearance normalized by allometric body weight was 0.88+/-0.16 l/h/kg3/4. The prehepatic extraction ratio was 0.51+/-0.13, and the hepatic extraction ratio was 0.24+/-0.04, the former explaining 95% of the variability in oral bioavailability (PT variant and the related haplotype c.1199G-c.1236C-c.2677G-c.3435C (P<0.05). The estimated oral dose requirement and clearance of cyclosporine remained largely unexplained by the genetic variations.Although these data suggest an age-related effect of ABCB1 polymorphism on oral bioavailability, further studies are required on the predictive value of genotyping for individualization of cyclosporine dosing in children.