Cystic islet cell neoplasms are among the rarest entities in the differential diagnosis of cystic tumor of the pancreas, and this malady raises difficult clinical problems. The diagnosis of insulinoma could be difficult if the functional activity is incomplete, which possibly leads to blunted symptoms of hypoglycemia and failure in the laboratory to provide evidence of hyperinsulinemia. Furthermore, if the imaging shows a smaller tumor than usual or an unusual morphology like cyctic tumor, then physicians can become somewhat confused. We report here on a clinical case of cystic insulinoma with the typical neuroglycopenic symptoms and laboratory-confirmed hyperinsulinemia. At resection, a 2-cm cavitary mass without central necrosis was excised and this was confirmed histologically as a purely cystic insulioma. This is the first report of a functional cystic insulinoma of the pancreas in Korea. We suggest that the differential diagnosis of endocrine tumor must be considered for any pancreatic cyst, and especially when it is discovered in a patient who is clinically suggestive of having the associated syndrome.