Idiopathic anaphylaxis (IA) is defined as anaphylaxis without any identifiable precipitating agent or event. The clinical manifestations of IA are the same as allergen-associated (immunologic) anaphylaxis and include urticaria, angioedema, hypotension, tachycardia, wheezing, stridor, pruritus, nausea, vomiting, flushing, diarrhea, dysphagia, light-headedness, and loss of consciousness. Patients usually tend to have the same manifestations on repeated episodes. IA is a prednisone-responsive disease that is ultimately a diagnosis of exclusion. Approximately 40% of patients are atopic. Serum tryptase (or urine histamine or its metabolite) will be elevated acutely, but, if elevated in the absence of anaphylaxis, should suggest alternative diagnoses, including indolent systemic mastocytosis. A focused history, examination, and follow-up will dictate whether a patient's symptoms may be attributable to disorders that mimic anaphylaxis, such as indolent systemic mastocytosis, carcinoid syndrome, pheochromocytoma, hereditary angioedema or acquired C1 esterase inhibitor deficiency, or panic attacks. The presence of urticaria may help limit the differential diagnosis because urticaria does not usually accompany any of the above-mentioned disorders, except for indolent systemic mastocytosis. IA is classified according to the symptoms as well as the frequency of attacks. Patients who experience six or more episodes in a year, or two or more episodes in 2 months are classified as having IA-frequent (IA-F). Patients who experience fewer episodes are classified as having IA-infrequent (IA-I). This distinction is important because patients with IA-F will initially require prednisone as disease-modifying therapy, whereas most patients who with IA-I will not require prednisone. Patients with IA must carry and know when and how to self-administer epinephrine.