Eighteen patients with Zollinger-Ellison syndrome were studied with 111In-pentetreotide SPECT in order to localize gastrinoma, the tumour responsible for this pathology. NMR imaging was also carried out. Eight patients were operated. 111In-pentetreotide was reinjected 4 hours before operation and the radioactivity of the excised tumours counted. The nature of the withdrawn tissues was assessed by immunohistochemistry (chromogranina A). The scintigraphy was repeated 3-6 months after surgery. 111In pentetreotide SPECT was more sensitive than NMR. It was also absolutely specific because all the radioactive tumours excised showed positive chromogranin A staining. The radioactivity/gram counted in gastrinomas exceeded 10 fold the hepatic and biliary radioactivity and 20-100 folds the radioactivity of blood and omentum. In all the operated patients but three, the scintigraphy performed after surgery did not detect tumours. However complete eradication did not occur, because though 3-6 months after surgery the gastrinemia was significantly lower with respect to pre-surgery results it did not return to normal values in all patients but two.