Mycophenolic acid (MPA) is glucuronidated primarily by uridine diphosphate glucuronosyltransferase enzymes (UGT) 1A9 and 1A8. These enzymes are highly polymorphic resulting in low activity and high expression phenotypes. We hypothesized that polymorphisms of UGT1A9 and 1A8 may alter MPA pharmacokinetics in kidney transplantation.One hundred seventeen kidney (n = 93), pancreas (n = 11), or simultaneous kidney and pancreas (SPK) (n = 13) transplant recipients were studied for the effect of UGT1A9 and UGT1A8 polymorphisms on MPA dose-corrected trough concentrations. Individuals were genotyped for UGT1A8 and UGT1A9 polymorphisms (1A8*2, 1A8*3, 1A9*3, 1A9-275 and 1A9-2152). Linear regression was used to estimate the effect of UGT polymorphisms on the individual's mean MPA dose-corrected trough concentration with and without stratification by calcineurin inhibitor. A multiple linear regression analysis was performed to assess the dependence between the average MPA dose-corrected trough concentration and age, gender, UGT genotype (1A8*2, 1A8*3, 1A9*3, 1A9-275, 1A9-2152), serum albumin, hemoglobin (Hgb), hematocrit (HCT), liver transaminases (AST, ALT), serum creatinine, and bilirubin.Mycophenolic acid dose-corrected trough concentrations were 60% higher in subjects heterozygous or homozygous for UGT1A8*2 than in those with the wild type (p = 0.02); however, this effect was dependent on concomitant calcineurin inhibitor. When subjects were stratified by calcineurin inhibitor status, the UGT1A8*2 effect was only apparent in the tacrolimus group (p A/-2152 C>T polymorphism who received cyclosporine (p A/-2152C>T polymorphism in the tacrolimus group.The effect of UGT1A8 and UGT1A9 variants on MPA metabolism appears to be modified by concomitant calcineurin inhibitor therapy. Confirmatory in vivo and in vitro studies are needed.