The anaphylaxis that is called idiopathic (A.I.) forms less than 1% of the publications that are concerned with anaphylaxis. The clinical picture associates all the symptoms of anaphylaxis, with particular frequency of Quincke's laryngeal oedema. A vital risk is supposed. No abnormal biological factor can be found. There is an associated, variable pathology in 20% of subjects, 58% are atopic. A.I. effects women more--69%. Quincke's hereditary angioneurotic oedema, the carcinoid syndrome, and the capillary hyperpermeability syndrome, paroxysm with monoclonal gammopathy, systemic mastocytosis must be eliminated as well as false anaphylaxis. The authors review the exceptional causes that may not be considered: drug anaphylaxis, to foods, hymenoptera, effort anaphylaxis, to hydatic antigens, to toboggans, to progesterone. Pathogenic hypotheses incriminate sensitization to unknown allergens, functional anomalies of mastocytes, heterogeneity of IgE. Addition of allergic and non-allergic factors is possible. Release of mediators other than histamine is one hypothesis proposed, to account for the inefficiency of anti H1. Prevention requires avoidance of aspirin, non-steroid anti-inflammatory drugs and beta blockers. Basic treatment is always corticosteroids, with anti H1 and sympathomimetic amines where the A.I. is severe.