BACKGROUND An occult insulinoma refers to a biochemically proven tumor with an anatomic site that remains indeterminate before operation. The amount of radiologic localization for such patients is debatable. METHODS Sixty-five patients with sporadic insulinomas were surgically treated at the Mayo Clinic between January 1980 and December 1990. True occult tumors were present in 31% of these patients (n = 20). Thirty-eight negative preoperative localization studies were performed, with 10 patients undergoing more than one study. A benign adenoma was found in 19 patients when they underwent exploratory operation, whereas one patient had malignant disease with hepatic metastases. Thirteen patients underwent intraoperative ultrasonography with a 7.5 MHz real-time high-resolution transducer. RESULTS Solitary lesions were successfully removed either by enucleation or by distal pancreatectomy in all 19 patients with benign disease. CONCLUSIONS This high success rate in the management of occult insulinomas suggests that extensive preoperative radiologic investigation is neither indicated nor cost-effective.