Conventional imaging modalities cannot localize the source of ACTH in 30-50% of patients with Cushing's syndrome (CS) caused by ectopic ACTH secretion (EAS). We prospectively evaluated whether [(18)F]fluorodeoxyglucose (FDG) positron emission tomography (PET) or [(111)In]-diethylenetriaminepentaacetate-D-Phe-pentetreotide (OCT) at higher than standard doses of radionuclide (18 mCi; H-OCT), can detect these tumors. Seventeen patients with presumed EAS based on inferior petrosal sinus sampling results underwent routine anatomical imaging studies [computed tomography (CT) and magnetic resonance imaging (MRI)] and OCT scintigraphy with 6 mCi (L-OCT). Research studies included FDG-PET in all patients and H-OCT if L-OCT was negative. ACTH-secreting tumors were localized in 13 patients and were occult in four. Nine of 17 CT, six of 16 MRI, six of 17 FDG-PET, eight of 17 L-OCT, and one of nine H-OCT studies were true positives. The sensitivity of CT and combined H- and L-OCT scintigraphy was higher (both 53%; 95% confidence interval, 29-76%) than that of MRI (37%; 95% confidence interval, 16-64%) or FDG-PET (35%; 95% confidence interval, 15-61%). FDG-PET did not detect tumors that were occult on CT/MRI. L-OCT was a useful complementary modality to CT and MRI. As H-OCT identified a tumor in one patient with otherwise negative imaging, it should be considered only when other imaging modalities fail to localize the ACTH-secreting tumor in patients with EAS.