A rare case of aberrant insulinoma in the duodenum is described. Hyperinsulinemia with typical hypoglycemic symptoms was induced by prolonged fasting. Selective angiography demonstrated a tumor supplied from the first branch of the jejunal artery, and duodenoscopy revealed a submucosal tumor at the third portion of the duodenum. An increase in venous plasma immunoreactive insulin concentration was evident in the vein draining from the tumor, by sampling through percutaneous transhepatic catheterization. Hypoglycemia was ameliorated after the removal of the submucosal tumor of the duodenum. Histologic and immunocytochemical characterization of the tumor showed an insulinoma, predominantly composed of cells with typical B-cell-like granules. The acid extract of the tumor contained 1.2 U/g of insulin, and this insulin, analyzed by reverse-phase high-pressure liquid chromatography, revealed that it had the same amino acid structure as that of human insulin.