The etiology of pathological jaundice can not be identified in almost half of the cases. The effect of promoter polymorphism in the UDP-glucuronosyltransferase 1A1 (UGT1A1) gene was investigated in healthy breast-fed Turkish neonates with unexplained and direct Coombs'-negative ABO incompatible hyperbilirubinemia. Newborns whose peak serum bilirubin levels were > or = 17 mg/dl and < or = 12.9 mg/dl within the first week of life formed the idiopathic hyperbilirubinemia (n: 50) and control (n: 54) groups, respectively. Thymine-adenine (TA) repeats in the promoter region of the UGT1A1 gene were investigated by polymerase chain reaction (PCR)-based non-radioactive DNA sequencing. In the idiopathic hyperbilirubinemia group, higher peak bilirubin levels, higher heterozygous and variant homozygous genotypes, higher TA7 allele frequencies, and shorter peak time were observed (p < 0.001, p < 0.001, p < 0.001, p < 0.05, respectively). In conclusion, healthy breast-fed Turkish neonates who carry heterozygous and variant homozygous genotypes in the UGT1A1 gene are at high risk of developing significant hyperbilirubinemia without additional icterogenic factors.