Neuroendocrine neoplasms (NENs) are a rare heterogeneous group of neoplasms that arise from neuroendocrine cells. Unknown-primary NENs (UP-NENs) are particularly challenging to diagnose and treat. Techniques such as immunohistochemical stains, functional imaging studies, and molecular cancer classifier assays may help clinicians identify the origin of a tumor. However, numerous medical facilities lack the necessary medical equipment, such as functional imaging scanning, to provide patients with a complete primary tumor survey. Even these tests are not enough to determine the original tumor in some cases. The present case series described the diagnosis and treatment outcomes of patients with UP-NEN in a single institution. The medical records of four patients treated between November 2012 and January 2022 were retrospectively reviewed and clinical symptoms, diagnostic methods, image findings and treatment modalities were considered. All patients were diagnosed having functional UP-NENs by using a short-acting somatostatin test. These patients were treated with long-acting release somatostatin analogs along with a positive result. Short-acting somatostatin is an alternatively simple method to determine if a patient has UP-NENs that are functional or expresses somatostatin receptors in the absence of imaging scanning.