We have performed percutaneous transhepatic portal catheterization (PTPC) with measurement of insulin concentration in 9 patients for 11 years, and could localize tumors in 8 of the 9 patients by the first PTPC. Even in the only case where we failed to localize a tumor by the first PTPC, we succeeded in localizing it with the second PTPC with 24 blood samples. In this case, only 1 blood sample drawn from the confluence of two veins, the dorsal pancreatic vein and splenic vein, showed an extremely high plasma insulin level (1,121 µU/ml), while the other 23 blood samples showed much lower insulin levels. In 2 of these 9 patients, the tumors were not palpable at operation, and PTPC was the only method that could localize them. This study indicates that PTPC is an excellent method for preoperative localization of insulinomas. Precise serial blood sampling covering the whole portosplenic venous system is indispensable to avoid a false-negative diagnosis