Insulinoma is the most common neuroendocrine tumor. Its clinical manifestations are frequently confounded with neuropsychiatric symptoms, and definitive diagnosis can be delayed for a long time. These tumors are usually small, of less than 2 cm. Thus, their preoperative localization is difficult. We report two patients with a clinical diagnosis of insulinoma, in whom the preoperative imaging study was negative. Both fulfilled diagnostic criteria, with high serum insulin levels in the presence of a blood glucose of less than 45 mg/dl. The imaging study, including abdominal computed tomography and pancreatic endoscopic ultrasonogrphy did not disclose the location of the tumor. A pancreatic angiography with selective stimulation with intra arterial calcium and venous sampling for insulin measurements, was performed in both patients. This test allowed the exact localization of the tumors and their successful excision. A review of other localization diagnostic tests is done.