Preoperative localization tests [sonography, computed tomography, angiography, percutaneous transhepatic portal venous sampling for insulin radioimmunoassay (PTP)] have a sensitivity of 60% to 90% in cases of organic hyperinsulinism. In all publications, however, the sensitivity of intraoperative localization, 75% to 100%, is distinctly higher. With the exception of PTP, all tumors that can be identified by preoperative localization can also be detected using palpation or intraoperative sonography. Preoperative localization diagnosis is therefore not absolutely necessary prior to primary operation in the case of organic hyperinsulinism. It is requested by many surgeons because: (1) only a few clinics have extensive experience with patients with an organic hyperinsulinism; (2) only a few clinics have extensive experience with intraoperative sonography; (3) the accuracy of palpation is improved following appropriate preoperative localization; and (4) in rare cases, the results of a PTP influence the operative procedure.