Sotos syndrome (OMIM #117550) is a congenital syndrome characterized by overgrowth with advanced bone age, macrocephaly, and learning difficulties. Endocrine complications of this syndrome have not yet been fully described in previous reports. We here investigated the clinical manifestations of Sotos syndrome in Japanese patients who presented with hyperinsulinemic hypoglycemia of infancy. We recruited patients diagnosed as having Sotos syndrome who presented with the complication of hyperinsulinemia during the neonatal period using a survey of the abstracts of Pediatric Meetings in domestic areas of Japan from 2007 to 2011. As a result, five patients (four females and one male) were recruited to evaluate the clinical presentation of Sotos syndrome by reference to the clinical record of each patient. A 5q35 deletion including the NSD1 gene was detected in all patients. Major anomalies in the central nervous, cardiovascular, and genito-urinary systems were frequently found. Hypoglycemia occurred between 0.5 and 3 hr after birth and high levels of insulin were initially found within 3 days of birth. The patients were treated with intravenous glucose infusion at a maximum rate of 4.6–11.0 mg/kg/min for 12–49 days. Three of the five patients required nasal tube feeding. One patient received medical treatment with diazoxide. This study shows that patients with Sotos syndrome may present with transient hyperinsulinemic hypoglycemia in the neonatal period. © 2012 Wiley Periodicals, Inc.