A 17-year-old female patient with features of epilepsy was treated with valproic acid. Two years later, hypoglycemia and hyperinsulinemia appeared. Transabdominal ultrasonography, spiral computed tomography, and indium-111 Octreoscan were performed without positive results. Endoscopic ultrasonography identified an oval tumor in the pancreatic tail with a color Doppler hypervascular pattern. Surgical enucleation decreased levels of insulin and C-peptide within 20 min, and the patient became free of symptoms and medications.