This study analyzes the clinical features of glucose-6-phosphate dehydrogenase (G6PD) deficiency in infants with marked hyperbilirubinemia. We retrospectively assessed a cohort of 413 infants with peak total serum bilirubin (TSB) level >or=20 mg/dL from 1995 to 2007. The prevalence of G6PD deficiency was proportional to the level of peak TSB: 21.1% (81/383) in 20 mg/dL to 29.9 mg/dL, 45.5% (10/22) in 30 mg/dL to 39.9 mg/dL, and 100% (8/8) in >or=40 mg/dL. Male sex was more common in G6PD deficiency (75.8%). When compared with G6PD-normal infants, those with G6PD deficiency tended to have extreme hyperbilirubinemia (peak TSB level >or=25 mg/dL) and hemoglobin value<13 g/dL (P<0.001). Furthermore, mortality rate was significantly higher in G6PD-deficient infants (3.0%) than in the G6PD-normal counterparts (0.0%). Among 58 of the G6PD-deficient infants who were followed for more than 12 months, 4 developed the classic neurologic manifestations of kernicterus (6.6%). These findings show that G6PD deficiency is an important risk factor of extreme hyperbilirubinemia, death, and kernicterus.