Increased bilirubin formation and decreased bilirubin conjugation play an important role in the pathogenesis of the newborn jaundice. Although physiologic jaundice is seen in most of the newborns, there are many risk factors that affect the severity and duration of hyperbilirubinemia. The latest studies showed that the frequency and severity of neonatal jaundice have been increased when mutations of the gene coding UDP-glucuronosyltransferase(UGT)1A1 coexist with other risk factors. Healthy term newborns weighing over 2500 g. were included in this study. The patient group consisted of 107 newborns either with total bilirubin level over 15 mg dl(-1) within 7 days or 5 mg dl(-1) after 15 days of age. The control group consisted of 55 newborns with bilirubin levels in physiological ranges. We investigated the frequency of promoter region [thymine-adenine(TA)]7 polymorphism in UGT1A1 gene. Factors which might cause pathologic and prolonged jaundice with coexisting polymorphism were also investigated. UGT1A1 6/7 genotype was found to be 11% in patient group and 13% in the control group. The difference between patient and control groups was not statistically significant. (TA)7 allele frequency was 0.069 and it is concluded that UGT1A1 promoter region polymorphism was not a risk factor for neonatal jaundice.