Six cases of malignant neuroendocrine tumours (3 bronchial, 1 ileocolic) are presented; diagnostic and clinical features, functional behaviour and the results of the treatment are discussed. Early functional manifestation (diarrhoea) was observed only in the first case of ileocolic tumour: in this patient the diarrhoea disappeared after the operation but further relapse occurred at the recurrence of the pathology. In the second ileocolic tumour diarrhoea with increased levels of 5 HT appeared later on, after the operation, expressing repeated disease. In one malignant bronchial carcinoid with early spinal secondary, the typical flushes appeared only after the explosion of metastatic spread. No functional disorders were observed in one patient with metastasis localised only in the bones. On the basis of actual knowledge the authors discuss the pathogenetic hypothesis of these events. Even though these experience limited, it may remark the importance of the quantity and quality of produced hormones, of the localization of metastasis in the liver and portal district and the unconstant production of clinical symptoms. The importance of A-chromogranin and of NSE as specific markers is discussed; the studies on the chromogranin have furthermore demonstrated the possible effect of the inhibitors of protonic pump on structural, hyper-dysplastic modification of gastric mucosa and this requires further investigation.