Nesidioblastosis, the process of differentiation of pancreatic islets from ductular epithelium, is a well-described cause of insulin-mediated hypoglycemia in neonates and infants, but not in adults. A 58-yr-old woman with characteristic clinical features of fasting hypoglycemia had inappropriately elevated plasma immunoreactive insulin levels during symptomatic episodes of fasting hypoglycemia. Angiography, palpation at laparotomy, and resection of the distal three-quarters of the pancreas provided no evidence of a tumor. Pathologic examination of the resected pancreas revealed the findings of nesidioblastosis, i.e., budding of islets from the wall of ductules, and also increased number and size of islets and abnormal shape and location of islets. An entire spectrum of islet cell abnormalities including nesidioblastosis can cause insulin-mediated hypoglycemia in adults, as it does in neonates and infants.