Aim. To estimate the effect of the reduced-function polymorphism SLCO1B1 c.521T>C on the renal graft function (estimated glomerular filtration rate, eGFR) over 12 months in patients treated with mycophenolic acid (MPA). Methods. Consecutive eligible adults (≥16 years of age) engrafted over a 6-year period who received MPA as a part of maintenance immunosuppression were assessed for eGFR on 9 occasions over 12 post-transplant months. The SLCO1B1 c.521C>T variant allele carriers (treated) and wild-type subjects (controls) were balanced on a range of demographic, medical, and genetic variables at baseline, and the development of eGFR (slope) was estimated with further adjustment for time-varying covariates. A subset of patients were assessed for exposure to MPA 5-7 days after the transplantation. Results. The adjusted eGFR slopes from day 1 to day 28 (peak), and from day 28 to day 365 were practically identical in treated (n=86) and control (n=168) patients (GMR=0.99, 95%CI 0.92-1.06, and GMR=0.98, 0.94-1.01, respectively). The rates of adverse renal outcomes and possible MPA-related adverse effects were low, and similar in treated and controls (adjusted RR=0.94, 0.49-1.84 and RR=1.08, 0.74-1.58, respectively). The pharmacokinetic substudy did not signal that treated and control patients differed with respect to MPA clearance, peak, trough or total exposure, overall (treated n=23, control n=45), if cotreated with cyclosporine (n=17 vs. n=26) or with tacrolimus (n=8 vs. n=17). Discussion. In patients treated with MPA, variant allele SLCO1B1 c.521T>C has no effect on the 12-month renal graft function. It does not seem to affect exposure to- and safety of MPA.