Congenital hyperinsulinism is the most frequent cause of persistent hypoglycemia in infancy. We present the case of a preterm, large-for-gestation-age infant with congenital hyperinsulinism who was found to have a novel p.Q392H homozygous mutation in the ABCC8 gene. The patient had severe brain damage, despite early diagnosis and appropriate management. The new mutations may provide an understanding of the prognosis and treatment of the disease. In addition, the data will help the family make informed decisions about future pregnancies.