Subtotal pancreatectomy was performed in a 6-month-old infant affected by a severe congenital hypoglycemia. Histologic examination disclosed clusters of beta cells scattered through the exocrine pancreas adjacent to small ducts. A microadenoma with cellular abnormalities was also noted. The beta cell nature of the nesidioblastosis was confirmed by electron microscopy. The infant is living and normoglycemic, although affected by cerebral impairment, 18 months after the surgical procedure. The findings in this patient emphasize the lack of features that may distinguish nesidioblastosis from endocrine adenoma of the pancreas.