Introduction: Part of the interindividual variability in response to drugs can be explained by Pharmacogenetics, correlating the presence of certain genetic variants, as Single Nucleotide Polymorphisms, SNPs, with blood drug levels, efficacy and adverse effects of the treatment. Cause-effect relationships are not always easy to detect, especially if there are also hidden effects of SNPs related to the concomitant drugs administered. Tacrolimus is mainly metabolized by CYP3A5 but an interaction with omeprazol has been described in a way that if its usual metabolizer, CYP2C19, shows reduced activity, then the non-metabolized omeprazol competes with tacrolimus for CYP3A5, and so the immunosuppressor blood levels are increased. Materials and methods: In 75 renal transplanted patients, treated with tacrolimus as calcineurin inhibitor and concomitant omeprazol, a panel of 36 SNPs located in the main drug transport and metabolism genes were analyzed. A single blood sample of each patient was employed for genotyping in the Sequenom MassArray device. The genotypes were correlated with tacrolimus levels expressed as trough level, Cmin/(dose/ weight), in the first two weeks after transplantation (in the 59 patients with levels data). Genotypes were also confronted with post-transplantation hospital stay duration (in days) and with allograft dysfunction. Those patients with abnormal tacrolimus metabolism due to CYP3A5 rs776746, GA/AA, were excluded. Results: The patients with AA variant in rs4244285 of CYP2C19 (n=4) showed a post-transplantation hospital stay of 30 days (median, range: 25-39) against 14,5 days (median, range: 7-46) of patients GG (n=51) + GA (n=20), (p=0.016, Kruskal-Wallis test). That difference was directly correlated with an increase in tacrolimus levels during the first 15 days post-trasplantation (p=0.001, Kruskal-wallis, median and range AA: 158; 117,2-279,0 vs. median and range GG+GA: 78,2; 35,0-144,0), also showing allograft dysfunction in the 4 AA recipients (acute tubular necrosis). Conclusions: The genotyping of CYP3A5 and CYP2C19 in renal transplantation recipients, treated with tacrolimus and omeprazol should be considered of interest, as AA variant in rs4244285 reduces the function of CYP2C19 which indirectly elicits an increase of tacrolimus blood levels, and with so, at least in our study population, the adverse effects described. Work partially supported by Consellería de Sanidad GE-039/11.