All 95 portal venous sampling (PVS) procedures performed in patients with Zollinger-Ellison syndrome in the past 10 years at the authors' institution were reviewed. It was possible to catheterize at least one branch of the pancreaticoduodenal venous arcade in all but two procedures (98%). The highest concentration of gastrin was found in a selective sample from the pancreaticoduodenal venous arcade or the transverse pancreatic vein in 56 of 91 procedures (62%). Selective sampling of pancreatic head veins yielded a gastrin gradient sufficient for localization in 60 patients (63%). Among 55 solitary sporadic gastrinomas identified at surgery, PVS allowed correct localization of the tumor in 32 (58%); if selective samples had not been obtained, only eight (15%) would have been localized (P less than .0005). Sensitivity was the same for tumors in the gastrinoma triangle (64%) and the body or tail of the pancreas (60%). There were no false-positive results. The overall complication rate was 20%, but most complications were abdominal pain lasting 3 days or less. Six patients (6%) had serious complications.