Mast cell activation syndrome (MCAS) is a condition with signs and symptoms involving the skin, gastrointestinal, cardiovascular, respiratory, and neurologic systems. It can be classified into primary, secondary, and idiopathic. Earlier proposed criteria for the diagnosis of MCAS included episodic symptoms consistent with mast cell mediator release affecting two or more organ systems with urticaria, angioedema, flushing, nausea, vomiting, diarrhea, abdominal cramping, hypotensive syncope or near syncope, tachycardia, wheezing, conjunctival injection, pruritus, and nasal stuffiness. Other criteria included a decrease in the frequency, severity, or resolution of symptoms with anti-mediator therapy including H(1) and H(2)histamine receptor antagonists, anti-leukotrienes, or mast cell stabilizers. Laboratory data that support the diagnosis include an increase of a validated urinary or serum marker of mast cell activation (MCA), namely the documentation of an increase of the marker above the patient's baseline value during symptomatic periods on more than two occasions, or baseline serum tryptase levels that are persistently above 15 ng/ml, or documentation of an increase of the tryptase level above baseline value on one occasion. Less specific assays are 24-h urine histamine metabolites, PGD(2) (Prostaglandin D(2)) or its metabolite, 11-β-prostaglandin F(2) alpha. A recent global definition, criteria, and classification include typical clinical symptoms, a substantial transient increase in serum total tryptase level or an increase in other mast cell derived mediators, such as histamine or PGD2 or their urinary metabolites, and a response of clinical symptoms to agents that attenuate the production or activities of mast cell mediators.