Anaphylaxis is an acute systemic hypersensitivity reaction with symptoms of immediate-type allergy, which involves particularly the skin, respiratory tract, cardiovascular system and gastrointestinal tract. The severity of anaphylactic reactions is variable but some are fatal. Hymenoptera venom anaphylaxis affects about 3% and food hypersensitivity 2.6-3.2% of the general population; drugs are the other frequent cause. Symptoms of anaphylaxis are characteristic, but none of them is obligatory--even urticaria is absent in about 10%--and each symptom may be found also in other conditions. Hence, there are numerous differential diagnostic considerations, and anaphylaxis may be overlooked in many cases. Diagnosis of anaphylaxis is based on the occurrence of characteristic symptoms, especially when they develop upon exposure to a potential trigger. It can be significantly supported by evidence of release of mediators in the course of the reaction. For clinical purposes, demonstration of an increase of mast cell tryptase serum concentration above the individual baseline value is useful. The correct diagnosis of anaphylaxis is not only important with regard to treatment of an acute reaction, but also for subsequent allergologic diagnostics and long-term management of the patient.