Since the description of the watery diarrhea syndrome by Verner and Morrison 29 years ago, clinical and experimental observations have elucidated the pathophysiology of this disease. Vasoactive intestinal polypeptide (VIP) is produced and released by a tumor of the pancreatic islets or by a tumor of neural crest origin such as a ganglioneuroma. Under normal conditions, current evidence suggests that VIP is a neurotransmitter in the central and peripheral nervous systems and particularly in the peptidergic nervous system. The low VIP plasma concentration observed in healthy subjects is viewed as a neuronal overflow since it has been impossible to ascertain any endocrine role for circulating VIP. Markedly elevated VIP plasma levels in the VIPoma syndrome lead to intestinal secretion with severe secretory diarrhea, resulting in hypovolemia, hypokalemia, and acidosis. These symptoms subside after successful tumor removal. Approximately 50 percent of patients have metastatic spread at the time of diagnosis. For these patients, a new and promising therapeutic modality is available in the form of a subcutaneously administered somatostatin analogue that relieves symptoms through potent inhibition of VIP release from tumor tissue.