A 40-year-old woman with recurrent episodes of hypoglycemia was referred because of suspected insulinoma. Prolonged fasting was discontinued after 24 h due to symptomatic hypoglycemia (29 mg/dl, glucose/insulin-ratio 0.34). Magnetic resonance tomography showed a small 0.3 cm lesion in the body of the pancreas. During subsequent surgery a pancreatic tumor could not be detected, neither by manual palpation nor intraoperative ultrasonography. A hyperinsulinemic, sequentially eu- and hypoglycemic clamp confirmed the biochemical diagnosis of endogenous hyperinsulinemia and intra-arterial calcium stimulation localized calcium responsive tissue in the feeding distribution of the superior mesenteric artery. An octreotide scan was negative. During relaparotomy, six weeks after the initial surgery, the pancreatic body and tail were resected and a ~1 cm nonmalignant insulinoma was found. Although the use of highly sensitive, and more sophisticated and expensive methods for the diagnosis and localization of insulinomas are not generally suggested, we recommend application of intra-arterial calcium stimulation if the tumor is not detected using conventional diagnostic procedures.