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 acquired C1 esterase inhibitor deficiency, or panic attacks. The presence of urticaria may help limit the differential because they do not usually accompany any of the aforementioned 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 IA-frequent (IA-F). Patients who experience fewer episodes are classified as IA-infrequent (IA-I). This distinction is important because IA-F patients initially will require prednisone as disease-modifying therapy whereas most IA-I patients will not. Patients with IA must carry and know when and how to self-administer epinephrine.