Promoter variants c.-3279T>G and A(TA)7TAA show decreased level of expression of UDP-glucuronosyl transferase 1A1 (UGT1A1) and consequently reduced activity of the enzyme catalyzing glucuronidation of bilirubin in hepatocytes. Thus, coincidental occurence of both variants should lead to increase of hyperbilirubinemia or contribute to its manifestation. In this study, investigation of both variants in 101 patients and 84 controls in a Caucasian population was performed and the results were compared with serum bilirubin levels. Despite high linkage disequilibrium between the loci (D' = 0.91, r(2) = 0.69), we have proven an interaction between the variants increasing the odds ratio for [(TA)7]+c.[-3279T>G] homozygotes to 54.2.