Since the 19th century, neuroendocrine neoplasms (NEN) have been identified. Right up to the present day, the nomenclature is still under debate reflecting the heterogeneity of these tumors. Although some of them are slow growing, some can be fearfully aggressive and may develop in almost any organ of the abdomen, thorax, neck, skin, and gonads. The most commonly observed sites of NEN are the lung and the gastroenteropancreatic system (GEP), where more than 50 entities have been observed. In case of a NEN of unknown primary tumor, the histopathological diagnostic workup includes immunohistochemistry for chromogranin A and synaptophysin, followed by specific tissue markers. Clinical presentation is very diverse, depending on the primary site and functionality of the tumor. In the case of the GEP-NEN, the main symptoms are abdominal pain, diarrhea, weight loss, gastrointestinal bleeding, or bowel obstruction. The presence of neuropsychiatric symptoms is not insignificant in this group of tumors. The authors report a case of a 51-year-old man who sought medical attention because of a three-month history of a consumptive disease. The diagnostic workup disclosed a diffuse nodular infiltration of the lungs, hypokalemia, and hypercalcemia in a cachectic patient. The clinical investigation could not proceed because of an infectious intercurrence, which led to the fatal outcome. Autopsy findings showed a diffuse metastatic NEN. The primary tumoral site could not be demonstrated with the available immunohistochemical panel.