Purpose : It has been known that breast milk cause prolonged unconjugated hyperbilirubinemia. UGT1A1 is a important gene of uridine diphosphate glucuronosyltransferase (UGT) which has a major role of bilirubin metabolism. These findings suggest that there is a relationship between UGT1A1 gene mutation and prolonged jaundice of breast feeding infant. The aim of study was to investigate whether a polymorphism of the UGT1A1 gene exist in prolonged hyperbilirubinemia of breast milk feeding Korean infant. Methods : The genomic DNA was isolated from 50 full term Korean neonates, who had greater than a 10 mg/dL of serem bilirubin after 2 weeks of birth with no significant cause, and the other genomic DNA was isolated from 162 full term Korean neonates of the control population. Both group fed breast milk. We performed direct sequencing of TATA box and Gly71Arg polymorphism of the UGT1A1 gene. Results : Two of the 50 neonates with hyperbilirubinemia had AA polymorphism, and 40 had GA polymorphism. Five of the 129 neonates of the control group had AA polymorphism, and 4 had GA polymorphism. The allele frequency of G>A polymorphism in the hyperbilirubinemia group was 44.0%; it was significantly higher than 5.4% of the control group. TATA box polymorpism was not different both group significantly. Conclusion : Our result indicated that Gly71Arg polymorphism is associated with the prolonged hyperbilirubinemia of breast milk-feeding infant in Korean, while TATA box polymorphism is not associated with the prolonged hyperbilirubinemia of breast milk-feeding infant in Korean.