To answer the open question whether a preoperative localization diagnostic is necessary in organic hyperinsulinism we prospectively investigated our 28 patients with an organic hyperinsulinism who were operated on between September 1, 1985 and December 31, 1994. Additionally we performed an analysis of the literature. In case of a solitary adenoma the preoperative localization diagnostic had a sensitivity between 28.4% (sonography) and 84% (endosonography). The combination of all preoperative localization procedures had a sensitivity between 70% (literature) and 90% (own patient material). With palpation and intraoperative sonography 97.1% (own patient material 100%) of all solitary adenomas could be found during first exploration. Multiple adenomas and reoperations have special problems. The pre- and intraoperative localization diagnostic failed in every fourth patient. In conclusion, in case of a primary operation without a MEN syndrome a preoperative localization procedure is not necessary due to the high sensitivity of the intraoperative diagnostic. In case of a MEN syndrome or reoperation a preoperative localization diagnostic is still advisable although the sensitivity is low.