Background Only one tumor site is usually biopsied to determine the histologic features of that patient’s entire tumor burden. We hypothesized that there are significant histologic and functional differences in primary neuroendocrine tumors (NETS) and their nodal or organ metastases. We also hypothesized that limited tumor sampling could lead to erroneous assumptions about the tumor’s histologic characteristics and clinical behavior. Materials and Methods Thirteen patients with metastatic well differentiated midgut NETS underwent simultaneous removal of their primary tumor, nodal metastasis, and organ metastasis. Each tumor site was stained quantitatively for Ki-67, chromogranin A (CGA), synaptophysin, CD31, and Factor VIII. Samples were also evaluated with in vitro tumor angiogenesis and drug chemoresistance assays. Results Ki-67 staining was nearly identical at all sites tested. Quantitative stains for CGA, synaptophysin, cluster of differentiation 31 (CD31), and Factor VIII varied considerably among the patient’s three tissue site samples. Only 6% of the tissue samples tested against a battery of chemotherapeutic agents exhibited susceptibility to a single drug at all three tumor sites. In contrast, several antiangiogenic agents exhibited uniform effectiveness across all three tissue sites in multiple patients. Conclusions Sampling only one NET tumor site may lead to erroneous assumptions about the tumor’s histologic features and functional behavior. Evaluation of primary tumors and their nodal and organ metastasis may be necessary to optimize clinical decision making.