TATA box abnormality in the promoter region of the bilirubin UDP glucuronosyltransferase 1 gene has been reported to cause Gilbert's syndrome in white subjects. It has also been reported that the majority of Japanese patients with Gilbert's syndrome are heterozygous for Gly71Arg in the coding region of this gene. On the other hand, some patients with chronic hepatitis often show signs of unexpected hyperbilirubinemia. The aims of this study were to determine which of the genetic variations, TATA box genotype or codon 71 genotype, is most closely related to serum bilirubin concentrations, and whether the TATA box genotype has an effect on serum bilirubin concentrations in patients with hepatitis C-associated chronic liver diseases. In a sample of 300 individuals selected from among the general Japanese population, mean concentrations of total serum bilirubin differed significantly among TATA box genotypes, but not among codon 71 genotypes. Concentration of total serum bilirubin was significantly correlated with TATA box genotypes. In 211 patients with hepatitis C-associated chronic liver diseases, mean concentrations of total serum bilirubin also differed significantly among TATA box genotypes. In patients with chronic hepatitis C, concentration of total serum bilirubin was significantly correlated with TATA box genotypes. In summary, TATA box genotypes, but not codon 71 genotypes, are closely related to serum bilirubin concentrations. TATA box genotypes should therefore be considered when evaluating hepatic function by serum bilirubin concentrations in cases of hepatitis C-associated chronic liver diseases.