Neonatal hyperbilirubinemia, which is prevalent among Asian peoples, has been considered as a physiological phenomenon, and its metabolic basis has not been clearly explained. Gilbert syndrome is a common inherited disease of unconjugated hyperbilirubinemia due to decreased bilirubin uridine diphosphate-glucuronosyltransferase (B-UGT), and its role in neonatal jaundice has recently been considered. We have previously reported that the Gly71Arg mutation of the B-UGT gene associated with Gilbert syndrome is prevalent in Japanese, Korean, and Chinese populations and was more frequently detected in neonates with severe hyperbilirubinemia than in control subjects. We have studied 159 Japanese full-term neonates, evaluating the relationship between the B-UGT genotype and the severity of jaundice, as assessed with a transcutaneous bilirubinometer. The gene frequency of the Gly71Arg mutation in these neonates was 0.19, and neonates carrying the Gly71Arg mutation had significantly increased bilirubin levels on days 2-4, manifested in a gene dose-dependent manner. The frequency of the Gly71Arg mutation was 0.47 in the neonates who required phototherapy (i.e., those with more severe hyperbilirubinemia), significantly higher than 0.16 in the neonates who did not require the therapy. The gene frequency of the TA repeat promoter polymorphism, the (TA)7 mutation, was 0.07, and neonates carrying this mutation did not have an increase in bilirubin. These results suggested that the Gly71Arg mutation contributes to the high incidence of neonatal hyperbilirubinemia in Japanese.