Two patients with clinical and biochemical evidences of islet cell hyperfunction underwent percutaneous transhepatic portal and pancreatic vein catheterization and hormone assays. Local step-ups of insulin level in the protal venous system, found in two patients with insulinoma, corresponded to tumor sites at surgery. One patient had single insulinoma and the other had malignant insulinoma and liver metastasis. Percutaneous transhepatic protal and pancreatic vein catheterization with measurement of radioimmunoactive insulin concentration is a safe and reliable method, and may play an important role in the localization of adenoma in patients with hyperinsulinism.