A comparative study of arteriography and pancreatic venous sampling (PVS) was performed in 55 patients. Twenty-seven patients with Zollinger-Ellison syndrome, 24 with insulinomas, and 4 control subjects underwent arteriography and PVS (for pancreatic hormonal radioimmunoassays) in an attempt to localize a suspected endocrine tumor. Accurate tumor localization was achieved by arteriography in 13% of the cases of gastrinoma and 29% of the cases of insulinoma. Although arteriographic signs could be described retrospectively in 62% of insulinomas, erroneous localization was relatively frequent. In contrast, localization by PVS was successful in 36 out of the 38 patients who underwent surgery; false-negative results were obtained in two patients, but in no case did PVS result in false localization. In 27% of insulinomas and 43% of gastrinomas, gross examination during surgery was negative but microscopic tumors were identified.