Many neuroendocrine tumors (NET) are small and may escape localization by conventional imaging techniques. In such cases, 11C-5-hydroxy-tryptophan (11C-5-HTP) positron emission tomography (PET) has been tested as an additional diagnostic tool. Nine patients with clinically, biochemically and/or histologically confirmed NET and negative computerized tomography (CT) or magnetic resonance imaging (MRI), and 111In-pentetreotide (Octreoscan) scintigraphy underwent imaging with 11C-5-HTP-PET/CT in order to: 1) detect the primary tumor lesion in three patients; 2) detect residual disease in two patients with appendiceal carcinoid, one with rectal carcinoid, one with midgut carcinoid, and one with ectopic ACTH secretion (EAS) due to residual pulmonary carcinoid; and 3) restage a patient with medullary thyroid carcinoma (MTC) and hepatic metastases. 11C-5-HTP-PET/CT detected lesions in the mediastinum in a patient with EAS due to a pulmonary carcinoid, further hepatic metastases in a patient with carcinoid syndrome (CS) from a NET of unknown primary, further hepatic metastases in the patient with MTC, and hepatic metastases in the patient with midgut carcinoid. The 11C-5-HTP-PET/CT findings contributed to radical cure of the patient with recurrent EAS, and pointed towards bilateral adrenalectomy in the patient with EAS without evident primary tumor. In addition, 11C-5- HTP-PET/CT directed towards combined surgical and medical treatment in the patient with CS and multiple rather than single hepatic metastases and in the patient with midgut carcinoid, and towards continuation of medical treatment in the patient with MTC. 11C-5-HTP-PET/CT is a useful imaging technique, providing additional information for the diagnosis, staging and decision-making regarding management of patients with NET.