A 6-day-old female newborn, readmitted for extreme hyperbilirubinemia with bilirubin encephalopathy, died despite 2 double-volume exchange transfusions. On autopsy examination the basal ganglia and hippocampus were selectively stained deep yellow. The infant was heterozygous for both the glucose-6-phosphate dehydrogenase Mediterranean mutation and for the (TA)(6)/(TA)(7) promoter polymorphism for the gene encoding the bilirubin conjugating enzyme uridine diphosphate-glucuronosyltransferase 1A1 (UGT1A1*28, associated with Gilbert syndrome). No additional mutations of the UGT1A1 were detected. Seemingly innocuous, heterozygotic mutations may interact synergistically to result in serious and even fatal outcomes.