UDP-glucuronosyltransferases (UGTs) represent a family of enzymes that glucuronidate many internal substances and drugs. This family acts as a drug metabolism phase II reactor in the liver and comprises one of the major protective mechanisms from toxic chemical substances. UGTs have two subfamilies; UGT1 and UGT2. UGT1 gene expresses 12 isoforms which are produced from a single gene by alternative splicing of a primary transcript. Each isoform has specific substrates. UGT1A1 conjugates bilirubin, and mutations of the gene cause hereditary unconjugated hyperbilirubinemias (Crigler-Najjar syndrome and Gilbert's syndrome). Recently, polymorphisms of UGT1A1 were revealed. In the Japanese population, there is a polymorphism of G71R and this mutation is a risk factor of neonatal hyperbiliruibnemia and a genetic cause of breast milk jaundice. These polymorphisms of UGTs might contribute to individual variations of drug metabolism and toxicity as well as inherited diseases.