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The interindividual variability of cyclosporin A (CsA) pharmacokinetics might be explained by heterogeneity in the cytochrome P450 3A (CYP3A) subfamily. Altered CYP3A enzyme activity was associated with variant allele of P450 oxidoreductase gene (POR*28). The aim of this study was to assess the impact of age, CYP3A5*3, CYP3A4*22, and POR*28 alleles on CsA pharmacokinetics in pediatric renal transplant recipients.Renal transplant patients receiving CsA (n = 47) were genotyped for CYP3A5*3, CYP3A4*22, and POR*28.CYP3A5 nonexpressers had higher overall dose-adjusted predose concentration (C0/dose; ng/mL per mg/kg) compared with expressers (31.48 ± 12.75 versus 22.44 ± 7.12, P = 0.01). CYP3A5 nonexpressers carrying POR*28 allele had a lower overall dose-adjusted concentration (C2/dose) than those with POR*1/*1 genotype (165.54 ± 70.40 versus 210.55 ± 79.98, P = 0.02), with age as covariate. Children aged 6 years and younger had a lower overall C0/dose (18.82 ± 4.72 versus 34.19 ± 11.89, P = 0.001) and C2/dose (106.75 ± 26.99 versus 209.20 ± 71.57, P < 0.001) compared with older children. Carriers of CYP3A5*3 allele aged ≤6 years required higher dose of CsA and achieved lower C0/dose and C2/dose, at most time points, than older carriers of this allele. Carriers of POR*28 allele aged ≤6 years required higher doses of CsA, whereas they achieved lower C0/dose and C2/dose, at most time points, in comparison to older carriers of this allele. The significant effect of age (P < 0.002) and CYP3A5 variation (P < 0.02) was shown for overall C0/dose, whereas age (P < 0.00001) and POR variation (P = 0.05) showed significant effect on C2/dose. Regression summary for overall C2/dose in patients aged 6 years younger showed a significant effect of both CYP3A5 and POR variations (P < 0.016).Younger age, POR*28 allele, and CYP3A5*3 allele were associated with higher CsA dosing requirements and lower concentration/dose ratio. Pretransplant screening of relevant polymorphisms in accordance with age should be considered to adjust therapy.