UDP-glucuronosyltransferases (UGTs) catalyze phase-II biotransformation reaction of a variety of substances. Among the UGT1A isoforms, UGT1A1, UGT1A3, UGT1A4, UGT1A6 and UGT1A9 are predominantly expressed in the liver. Interindividual variability in expression of these isoforms would cause interindividual differences in drug response, toxicity and cancer susceptibility. In the present study, we investigated the interindividual variability in UGT1A mRNA expression and whether hepatocyte nuclear factor 1alpha (HNF1alpha) and HNF4alpha were factors responsible for their variability in human livers. The amounts of UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A9, HNF1alpha and HNF4alpha mRNA in 18 human livers were measured by quantitative real-time polymerase chain reaction. The largest and smallest interindividual differences in expression levels were observed in UGT1A1 (8.6-fold) and UGT1A4 (2.5-fold) mRNA, respectively. The amounts of HNF1alpha and HNF4alpha mRNA were strongly correlated with the amount of UGT1A9 mRNA and moderately correlated with that of UGT1A6 mRNA, whereas no significant correlation was found with the amounts of UGT1A1, UGT1A3 and UGT1A4 mRNA. Our results suggest that HNF1alpha and HNF4alpha are the factors involved in the interindividual variability of UGT1A6 and UGT1A9 mRNA expression. Further studies of other transcription factors are needed to clarify the factor(s) determining the interindividual variations in UGT1A1, UGT1A3 and UGT1A4 mRNA expression.