In patients with the Zollinger-Ellison syndrome, which is either sporadic or integrated into multiple endocrine neoplasia type 1, accurate localization of all the tumours is difficult and may have therapeutic implications. In an attempt to improve this localization, somatostatin receptor scintigraphy using [111In-DTPA-D-Phe1]-octreotide was performed prospectively in 48 consecutive patients with the Zollinger-Ellison syndrome. Thirty of them had the sporadic type of this disease. Scintigraphic data were compared with data obtained by conventional imaging methods, and also, in 32 selected patients, with those obtained by endoscopic ultrasonography. Somatostatin receptor scintigraphy showed abnormal tracer uptake in 39 patients (81%), in whom it correctly identified 50 of the 60 tumoral sites (83%) previously localized by the other imaging methods. In 17 patients (35%) somatostatin receptor scintigraphy disclosed abnormal tracer uptake at 18 different tumoral sites: 14 were located in the abdomen, including four in the liver and eight in the duodenopancreatic area, and four outside the abdomen, including two in the mediastinum. Six of the ten tumoral sites which were not correctly identified by somatostatin receptor scintigraphy were located in the duodenopancreatic area. However, in the 20 patients for whom conventional techniques failed to visualize any tumour in the duodenopancreatic area, somatostatin receptor scintigraphy was positive in ten (50%) whereas endoscopic ultrasonography was only positive in five (25%). In our patients with the Zollinger-Ellison syndrome, somatostatin receptor scintigraphy appeared to be a useful new addition to the battery of tests used for tumour detection.