A 76-year-old woman suffered from somnolence while fasting for almost 2 years. Fasting plasma glucose (FPG) (40 mg/dl) and the immunoreactive insulin (IRI) level (8.8 microU/ml) were not compatible with Fajan's ratio or Turner's ratio observed in typical insulinoma. The secretin test showed no response to insulin secretion, suggesting the presence of insulinoma. Abdominal dynamic computed tomography (CT) revealed a 12-mm hypervascular lesion in the head of the pancreas. A selective arterial calcium infusion test (SACI) was performed, during which IRI in the hepatic venous blood was measured following selective intraarterial calcium infusion. An increase in IRI levels in the gastroduodenal and superior mesenteric arteries suggested the presence of a functional insulinoma in the head of the pancreas. Enucleation of the tumor improved FPG and IRI levels to 138 mg/dl and 3.8 microU/ml, respectively. After surgery, a secretin test showed a 5-fold increase in IRI levels, suggesting normal beta cell function. This case illustrates the value of the secretin test for the diagnosis of insulinoma and for the postoperative assessment of beta cell function. It further illustrates the value of the SACI for localizing an insulinoma.