Transhepatic portal vein catheterization for localization of insulinomas: a ten-year experience. One of the most important factors in the management of insulinomas is the ability to localize the tumor accurately either before or during surgery. We prospectively carried out transhepatic portal venous sampling (THPVS) for tumor localization in 35 of 40 patients with organic hyperinsulinism during a 10-year period. In 32 patients who underwent THPVS and in whom a single tumor was subsequently identified surgically, the maximal insulin gradient was located in the vicinity of the tumor in 100% of cases. Specific regionalization of the tumor on the basis of the site of the maximal insulin gradient to one of three regions (the tail, the body/neck region, and the head/uncinate region) gave a sensitivity of 81% and a specificity of 91%. In contrast, the use of specific cutoff levels for the insulin gradient as a guide to the presence of a tumor in one of these three regions did not increase the accuracy, leading instead to a significant loss of sensitivity with no comparable increase in specificity. There were no major complications from the procedure in any patient. The initial use of computed tomographic/ultrasound scanning and selective angiography localized only 46% of tumors, whereas subsequent THPVS led to the accurate preoperative localization of 100% of all tumors submitted to surgery. Although the surgeon would have identified 81% of the tumors correctly at operation, in 19% (n = 6) he would have failed. Four tumors were in the uncinate and two were in the head. It seems that in patients with proved or established organic hyperinsulinism, THPVS may continue to be of value, if only to regionalize the tumor, especially those in the pancreatic head and uncinate process so as to preclude noncurative operations on the body and tail of the pancreas. 