It is often difficult to localize gastrinomas in patients with Zollinger-Ellison syndrome (ZES). We have developed a new useful method, the selective arterial secretin injection test (SASI test), for localizing gastrinomas in patients with ZES. The SASI test first determines the arteries feeding the gastrinomas and then locates the gastrinomas. In 12 patients with ZES, the SASI test clearly localized the gastrinomas, while results obtained with computed tomography or portal venous blood sampling had a positive predictability of less than 10%. Following localization with the SASI test, curative resection of gastrinomas was successfully performed on 7 patients who consented to the operation. Each of 3 patients had one duodenal submucosal microgastrinoma and one or more metastatic lymph node; the other 4 patients had 2 to 10 microgastrinomas or large gastrinomas in the duodenum or the pancreas. All 7 patients, who exhibited negative responses with a postoperative secretin test, have been completely cured from 3 months to 4 years postoperatively. The usefulness of the SASI test for preoperative evaluation is demonstrated by the fact that it gives a 100% positive predictability rate as well as a 100% negative predictability rate. Hence the SASI test precisely locates functioning gastrinomas.