Idiopathic anaphylaxis is a prednisone-responsive condition without external cause, but it can coexist with food-, medication-, or exercise-induced anaphylaxis. Mast cell activation may occur at night or after foods that have been eaten with impunity many times previously. Idiopathic anaphylaxis can be classified into frequent (if there are six or more episodes per year or two episodes in the last 2 months) or infrequent (if episodes occur less often). Idiopathic anaphylaxis-generalized consists of urticaria or angioedema associated with severe respiratory distress, syncope or hypotension, and gastrointestinal symptoms. Idiopathic anaphylaxis-angioedema consists of massive tongue enlargement or severe pharyngeal or laryngeal swelling with urticaria or peripheral angioedema. The differential diagnosis of idiopathic anaphylaxis is reviewed, and treatment approaches are presented.