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Insulinoma in a patient with pre-existing diabetes is extremely rare. A 74-year-old woman with type 2 diabetes mellitus who had been treated with a sulfonylurea for 6 years began experiencing frequent episodes of hypoglycemia. Endogenous hyperinsulinism was found 9 months after the sulfonylurea was discontinued, and transabdominal ultrasonography and magnetic resonance imaging identified a pancreatic tumor. Pathology examination of the resected tumor demonstrated an insulinoma. Postoperatively, the patient had no further episodes of hypoglycemia. Thereafter, she required insulin to control her hyperglycemia. Although hypoglycemic agents are the commonest cause of hypoglycemia in type 2 diabetes, insulinomas may occur in these patients. This possibility should be considered if the hypoglycemia persists despite dose adjustment or cessation of the drugs.