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This article is a review of the historical, biochemical, and functional aspects of the enzyme diamine oxidase (DAO). The amine oxidase DAO, formerly called histaminase, is found in various tissues, but is especially active in the intestinal mucosa. Its function is the oxidative deaminating of several polyamines, essential substances for cell proliferation. DAO is thus a regulating enzyme in rapidly proliferating tissues such a bone marrow and intestinal mucosa. Results from several studies have demonstrated that both ornithine decarboxylase (ODC) and DAO activity rise during adaptive hyperplasia seen after small bowel resection. The ODC-dependent increase in polyamine content and subsequent increase in cell proliferative activity is probably downregulated locally in the villus tip by the increased DAO activity. DAO is normally present in very small amounts in the circulation and its basal plasma levels are positively correlated with the maturity and integrity of the intestinal mucosa. After intravenous administration of heparin, DAO is released from its capillary binding sites in the lamina propria into the peripheral circulation. Measurement of postheparin DAO release enhances its sensitivity and is now extensively studied to assess its value as follow-up or screening test for several enteropathies. Measuring basal as well as postheparin DAO levels has potential relevance following small bowel transplantation. Rejection of the small bowel graft leads to mucosal damage, which could conceivably lead to changes in DAO activity.