Patients previously designated as having chronic idiopathic urticaria are now divided into 2 groups: 40% to 50% with chronic autoimmune urticaria, and the remainder with chronic idiopathic urticaria. Patients in both groups may have concomitant angioedema (approximately 40%). The autoimmune subgroup has an association with antithyroid antibodies and is caused by IgG antibody to the alpha subunit of the IgE receptor (35% to 40%), usually reactive with unoccupied IgE receptors, or IgG antibody to IgE (5% to 10%). Complement activation augments histamine secretion by release of C5a. The IgG subclasses that appear to be pathogenic are IgG(1), IgG(3), and, to a lesser degree, IgG(4), but not IgG(2). Histology of chronic urticaria (both subtypes) reveals a perivascular non-necrotizing infiltrate of CD4(+) lymphocytes consisting of a mixture of T(H)1 and T(H)2 subtypes, plus monocytes, neutrophils, eosinophils, and basophils. These cells are recruited as a result of interactions with C5a, cell priming cytokines, chemokines, and adhesion molecules. Suggested therapy for patients with severe disease involves the use of high-dose hydroxyzine or diphenhydramine when nonsedating antihistamines are ineffective, supplemented by H-2 antagonists and leukotriene antagonists. The most severe patient may require protracted treatment with low-dose alternate-day steroid or cyclosporine. Cyclosporine can be steroid-sparing when side effects are encountered or when use of steroids is relatively contraindicated. Careful monitoring of blood pressure, BUN, creatinine, and urinalysis is required.