Abstract: Functional techniques are playing a pivotal role in the imaging of cancer today. Our aim was to compare, on a head-to-head basis, 3 functional imaging techniques in patients with histologically verified neuroendocrine tumors: somatostatin receptor scintigraphy (SRS) with 111Indiethylenetriaminepentaacetic acidoctreotide, scintigraphy with 123I-metaiodobenzylguanidine (MIBG), and 18F-FDG PET. Methods: Ninety-six prospectively enrolled patients with neuroendocrine tumors underwent SRS, 123I-MIBG scintigraphy, and 18F-FDG PET on average within 40 d. The functional images were fused with low-dose CT scans for anatomic localization, and the imaging results were compared with the proliferation index as determined by Ki67. Results: The overall sensitivity of SRS, 123I-MIBG scintigraphy, and 18F-FDG PET was 89%, 52%, and 58%, respectively. Of the 11 SRS-negative patients, 7 were 18F-FDG PET-positive, of which 3 were also 123I-MIBG scintigraphy–positive, giving a combined overall sensitivity of 96%. SRS also exceeded 123I-MIBG scintigraphy and 18F-FDG PET based on the number of lesions detected (393, 185, and 225, respectively) and tumor subtypes. 123I-MIBG scintigraphy was superior to 18F-FDG PET for ileal neuroendocrine tumors, and 18F-FDG PET was superior to 123I-MIBG scintigraphy for pancreaticoduodenal neuroendocrine tumors. The sensitivity of 18F-FDG PET (92%) exceeded that of both SRS (69%) and 123I-MIBG scintigraphy (46%) for tumors with a proliferation index above 15%. Conclusion:The overall sensitivity of 123I-MIBG scintigraphy and 18F-FDG PET was low compared with SRS. However, for tumors with a high proliferation rate, 18F-FDG PET had the highest sensitivity. The results indicate that, although SRS should still be the routine method, 18F-FDG PET provides complementary diagnostic information and is of value for neuroendocrine tumor patients with negative SRS findings or a high proliferation index.