Intraoperative ultrasonography is more sensitive than preoperative and other intraoperative techniques for localizing insulinoma.Retrospective review.A tertiary referral center.All patients with a biochemical diagnosis of organic hyperinsulinism who were referred to University of California, San Francisco, from 1975 to 1998.Sensitivities of the localization techniques for insulinoma were evaluated.The sensitivities of tumor localization with arteriography, computed tomography, preoperative ultrasonography, magnetic resonance imaging, magnetic resonance imaging with gadolinium, transhepatic venous sampling, palpation, and intraoperative ultrasonography were 47%, 24%, 50%, 30%, 40%, 55%, 76%, and 91%, respectively. Nine of the 11 nonpalpable and nonvisible tumors at operation were localized by intraoperative ultrasonography.The currently available preoperative localization tests are not reliable enough to be recommended when intraoperative ultrasonography is available.