Core tip: Mast cell activation disease (MCAD) is characterized by accumulation of genetically altered mast cells and/or abnormal release of these cells’ mediators, affecting functions in potentially every organ system, often without causing abnormalities in routine laboratory or radiologic testing. Recent data suggest a high prevalence of MCAD. Thus, MCAD should be considered routinely in the differential diagnosis of patients with chronic multisystem polymorbidity of a generally inflammatory theme or patients in whom established diagnoses do not well account for the patient’s presentation of symptoms consistent with mast cell mediator release. Mediator testing can be challenging but typically is manageable. Diagnostic efforts are worthwhile, as diagnosis often leads to effective therapy.