Gastroenteropancreatic endocrine tumors are difficult to localize. At the same time the tumor is localized, though, there is an opportunity for cure or to remove tumor tissue. In this study we have prospectively examined the ability of 111In-octreotide scintigraphy, magnetic resonance imaging (MRI), and computed tomography (CT) to localize tumor lesions in 24 patients with a biochemical or histologic diagnosis of neuroendocrine tumor. In eight patients a surgical assessment of the imaging results was prospectively performed. Planar and abdominal single-photon emission tomography (SPET) images acquired 4 and 24 hours after 180 to 220 MBq of 111In-octreotide injection were evaluated and compared with conventional imaging techniques. SPET scintigraphy visualized more presumed tumor lesions (n = 39) than conventional imaging studies (MRI,n = 25; CT,n = 13); 23 of 24 patients had positive octreotide scintigraphy, 17 of 24 had positive MRI-scans, and 12 of 24 patients had positive CT scans. It was concluded that 111In-octreotide scintigraphy combined with conventional imaging improves the preoperative localization of presumably tumorous lesions in patients with gastroenterohepatic endocrine tumors.