The role of the enzyme diamine oxidase (DAO) in the diagnosis of histamine allergy and intolerance have remained unclear until recently and has not attracted the attention of scientists. Not long ago, the number of publications has been steadily increasing. In reliable clinical trials, a significant reduction in all histamine intolerance-related symptoms has been proved, thanks to oral DAO supplementation (from pork kidneys), and recently the experimental use of a recombinant humanized rhDAO molecule with potentially wide therapeutic uses from anaphylaxis and asthma to urticaria and atopic dermatitis has been described. The syndromes of applying histamine intolerance and allergies are discussed. DAO measurement may be helpful in assessing the severity of respiratory allergy, represent a new biomarker for predicting the effectiveness of specific allergen immunotherapy; dietary intervention (low-histamine diet), and the use of H1 and H2 receptor antagonists.