Both cyclosporine and tacrolimus display a narrow therapeutic index as well as high interindividual pharmacokinetic variability. We approached the effect of the CYP3A4*18B and CYP3A5*3 polymorphisms and haplotypes on the whole blood cyclosporine or tacrolimus concentration in Chinese renal transplant patients during the first month after transplantation.A total of 83 recipients receiving tacrolimus or cyclosporine was genotyped by polymerase chain reaction-restriction fragment length polymorphism (PCR-RFLP). The whole blood concentration was measured by enzyme-multiplied immunoassay technique.Both CYP3A4*18B and CYP3A5*3 polymorphisms affected the tacrolimus dose-adjusted trough concentration (C0/D). The tacrolimus C0/D was higher in carriers of haplotype GG compared with the non-carriers. The cyclosporine dose-adjusted 2-hour post-dose concentrations (C2/D), dose-adjusted C0 + C2 ((C0 + C2)/D) and C2/C0 during Days 15 - 21 displayed significant difference among the three genotypes. Statistical difference was observed between CYP3A4*1/*1 and CYP3A4*18B/*18B groups and between CYP3A4*1/*18B and CYP3A4*18B/*18B groups, but no difference was detected between CYP3A4*1/*1 and CYP3A4*1/*18B groups. No difference was found in C0/D among the three genotypes of CYP3A4*18B polymorphism, and neither CYP3A5*3 polymorphisms nor CYP3A haplotype-derived genotypes affected the cyclosporine dose-adjusted concentration.Genetic polymorphisms of CYP3A5*3 and CYP3A4*18B may be partly responsible in large interindividual variability of cyclosporine and tacrolimus blood levels in Chinese renal transplant patients during the first month after transplantation. A patient carried combined genotype of CYP3A4*1/*1-CYP3A5* 3/*3 might require lower tacrolimus doses to achieve target concentration levels. Genotyping of CYP3A4*18B and CYP3A5*3 before transplantation is of benefit in determining a suitable initial dose for each patient.