Complement may play a primary or exacerbating factor in the production of urticaria and/or angioedema. As we have seen, the absence of inhibitor proteins, such as ClINH, C3bINA, or Carboxypeptidase N Anaphylatoxin Inactivator) can permit small amounts of active complement fragments, liberated "normally" through the action of nonspecific effector pathways, to produce generalized urticaria or angioedema. Abnormal degrees of complement activation, produced by circulating immune complexes, cryoglobulins, paraproteins, may also cause angioedema and/or urticaria. Autoimmune diseases, lymphoreticular malignancy, infections, drug reactions, and syndromes currently termed idioipathic may be associated with urticaria and angioedema due to pathologic complement activation. Direct effects of complement on blood vessels, either by anaphylatoxins or by the deposition of complement fixing immune complexes, may produce the vascular permeability requisite to these syndromes. Secondary effects, produced by the recruitment of inflammatory cells and/or the liberation of other vasoactive mediators from these cells may also occur. Finally, complement may act in a potent synergistic fashion with mediators such as prostaglandins to produce vascular permeability. Current studies, using antigenic, hemolytic, and immunofluorescent techniques for detecting complement involvement have identified a relatively limited number of syndromes in which complement is involved. It is anticipated that the use of more sensitive assays for anaphylatoxins, such as radioimmunoassay, neutrophil aggregation, and others, coupled with an understanding of the potential synergism between these and other mediators, will lead to the recognition of a wider role for complement in urticaria and angioedema.