Chronic idiopathic urticaria has long been a demoralizing disease, baffling allergists and dermatologists alike, to the detriment of the patient. Recent findings, however, have shed light on causation in many, though not all, of these patients. The purpose of this review is to bring the reader up to date on the current position regarding aetiology and pathogenesis and the strength of the evidence. The review also seeks to point up rational approaches to diagnosis and treatment in the light of these developments.Chronic idiopathic urticaria encompasses at least two subgroups. One of these is the now well-established entity of autoimmune chronic urticaria, due to autoantibodies against either the high-affinity IgE receptor Fc epsilon R1 or, less commonly, IgE. These patients, who co-segregate with chronic idiopathic urticaria patients having an increased frequency of antithyroid autoantibodies, represent 30-50% of the patients previously designated as having chronic idiopathic urticaria. Convenient routine diagnostic tests for this subset remain elusive. The remaining 50% of patients with chronic idiopathic urticaria remain truly 'idiopathic', although the condition in some may have an autoimmune basis, autoantibodies having eluded current techniques for detection. Selected patients with autoimmune urticaria may benefit from immunotherapy.It is now known that in 30-50% of patients with chronic idiopathic urticaria, the condition has an autoimmune basis, although confirmation of the diagnosis in these patients is not straightforward. In selected patients, attempts to establish this diagnosis are worthwhile since there are important therapeutic implications.