Introduction: Hyperinsulinemic hypoglycemia of infancy (HHI) is a rare condition, but emergency condition can cause irreversible brain damage at neonatal period. HHI is known genetics of focal islet-cells hyperplasia associated with a paternally mutation of the ABCC8 or the KCNJ11 genes. We report here in a case of HHI with a novel mutation Case: A 1-day old girl weighing 4,100 g (90 percentile) born at 40+6 weeks of gestation was transferred to ours hospital with repetitive seizure-like behaviors and apneic events. Her mother did not have any medical problems including gestational diabetes mellitus. On admission, she presented repetitive apneic events and lethargy. Her laboratory findings showed non-ketotic hypoglycemia (serum glucose 2 mg/dL). Serum Insulin level was 9.1 mIU/L despite of hypoglycemia and there was no evidence of fatty acid oxidation disorder. Genetic analysis for ABCC8 was performed and c.4608G＞A on exon 38 was noted. We treated her by diazoxide but clinical response was poor. Conclusion: HHI is a one of critical disorder causing irreversible brain damage. So, early detection and treatment are needed. We report a novel mutation of ABCC8 in a Korean patient with hyperinsulinemic hypoglycemia.