P-glycoprotein (P-gp) and the drug metabolizing enzymes have major pharmacokinetic effects. Variability in tacrolimus absorption is influenced by P-gp activity which, in turn, is affected by single nucleotide polymorphisms (SNPs) within the multidrug resistance-1 gene (MDR-1). Tacrolimus dose requirements of 206 stable renal transplant patients were related to MDR-1 genotypes of SNPs C1236T, G2677T/A and C3435T, as well as haplotypes: C-G-C and T-T-T. Lower dose-normalized blood tacrolimus concentrations were achieved for: 2677-GG genotype patients, as compared to 2677-TT, and for 3435-CC patients as compared to 3435-TT patients. There was a small, but significant, difference in dose requirements between haplotypes C-G-C and T-T-T patients, which was not significant when patients were subclassified as producers and non-producers of cytochrome P450 3A5 (CYP3A5). The activities of CYP3A5 and P-gp have been shown to influence bioavailability of several drugs. Our data suggest that MDR-1 haplotypes have a relatively minor association with tacrolimus pharmacokinetics.