Persistent hyperinsulinemic hypoglycemia is caused most commonly by an insulinoma in adults or by nesidioblastosis in neonates. In adults, nesidioblastosis is a rare disorder characterized by diffuse or disseminated proliferation of islet cells. We recently encountered a case of nesidioblastosis in an adult. A 71-year-old man was admitted due to intermittent general weakness, abdominal pain, and mild dyspnea. The patient underwent a subtotal gastrectomy for a gastric adenocarcinoma two years ago. After 5 d of admission, the patient showed symptoms of cold sweating, chilling, and hypotension 30 rain after eating. Thereafter,he frequently showed similar symptoms accounting for hypoglycemia regardless of food consumption. Laboratory findings revealed a low fasting blood glucose level (25 mg/dL), and a high insulin level (47 μIU/mL). Selective intra-arterial calcium stimulation with hepatic venous sampling (ASVS) was performed to localize a mass and revealed an increased insulin level about four-fold that of the normal fasting level at 60 s in the splenic artery, which suggested the presence of an insulinoma in the tail of pancreas. A distal pancreatectomy was performed. Neither intraoperative exploration nor a frozen biopsy specimen detected any mass-forming lesion. On the histological examination, many of the islets were enlarged and irregularly shaped in all specimens, the arrangement of which was a Iobulated islet pattern. Cytologically, a considerable subpopulation of endocrine cells showed enlarged and hyperchromatic nuclei. By immunohistochemistry, the cells were identified as β-cells. These clinical, radiological, microscopic and immuno-histochemical findings are consistent with diffuse nesidioblastosis in adults.