Purpose The use of immunosuppressive (IS) drugs is characterized by a narrow therapeutic window and high individual variability. Genetic variation has been shown to influence the response to IS. The aim of this study was to characterize the allele frequencies of candidate polymorphisms in the PK and PD pathways of IS [tacrolimus (Tac) and cyclosporine (CS)]. The associations between the polymorphisms, the drug blood trough levels (TL) and graft rejection (GR) after heart Tx (HTx) was also evaluated. Methods and Materials A cohort of 107 HTx patients (pts) were recruited to the study. Candidate polymorphisms were genotyped using TaqMan® genotyping assays. 52 Tac-based and 42 CS-based pts were included in the analyses. Results Significant differences in the mean dose-adjusted Tac TL were observed between CYP3A5*3 genotype groups at 1, 3, and 6 months after HTx. Carriers of CYP3A5 expressor genotype (*1/*3) required higher daily doses to reach the target TL compared to CYP3A5 nonexpressors [ figure 1 ] (*3/*3). PXR g.-25385 T variant influenced Tac dose requirement early (month 1) after HTx, while POR*28 variant influenced it later (month 3 and 6). However, no correlation between the genotype CYP3A5 variants and biopsy-proven GR was detected. In the CS group we found only one difference for the CYP3A5 variants at 6 months post HTx (p=0.043). Conclusions Carriers of CYP3A5 expressor genotype (*1/*3) required significantly higher daily Tac doses to reach the target TL at all studied time points compared to CYP3A5 nonexpressors (CYP3A5*3/*3). The POR*28 and PXR g.-25385 T variants may be additional modulators of the pharmacokinetics of Tac. The Caucasian HapMap reference supports our data.