Background.Cyclosporine A (CsA) is a critical immunosuppressive drug with a narrow therapeutic range and wide interindividual variation in its pharmacokinetics. Many factors, including P-glycoprotein (PGP), influence the oral bioavailability and interpatient variability of CsA. A number of polymorphisms have been identified in the humanMDR1gene, and some of them have been found to be associated with an altered expression of PGP. We have investigated the role of these polymorphisms in CsA absorption from kidney transplant recipients. In addition, we also investigated the effect of amlodipine on CsA absorption.Methods.The area under the time-concentration curve from 0 to 2 hr (AUC0–2) estimated by the trapezoidal rule was used for the evaluation of extent of CsA absorption. The genotypes were identified by a polymerase chain reaction, restriction fragment length polymorphism analysis.Results.No association was found between polymorphisms in theMDR1and CsA AUC0–2/dose/kg. In contrast, the combination of amlodipine significantly increased CsA AUC0–2/dose/kg (706.2 &mgr;g·hr/L to 819.2 &mgr;g·hr/L,P<0.05). Furthermore, we attempted to compareMDR1polymorphisms and the absorption of CsA again without patients receiving amlodipine, but there was still no significant difference.Conclusions.There is no relationship between polymorphisms forMDR1and CsA absorption, suggesting polymorphisms forMDR1cannot account for the interpatient variability of CsA. Amlodipine, which is the substrate of PGP, significantly increased CsA absorption. These results indicate that PGP plays a significant role in CsA absorption, but its polymorphisms could not influence the CsA absorption.