W hile it is well established that acute allergic urticaria is caused by degranulation of skin mast cells occurring after allergen/IgE-dependent cross-linking of high affinity IgE receptors (FcepsilonRI), the pathophysiologic mechanisms operative in chronic urticaria (CU) are less well understood. Some evidence points to the existence of histamine-releasing activity in the serum of CU patients which possibly acts via triggering of FcepsilonRI. In this study, we aimed to better characterize this anti-FcepsilonRIalpha reactivity of CU patients using affinity-purified, IgE-depleted IgG fractions of such individuals (CU-IgG). Using immobilized, recombinant soluble FcepsilonRIalpha as a a reaction target for Western blot studies, we found that 12/32 (37%) CU-IgG serum samples exhibited IgG autoreactivity against FcepsilonRI- alpha. These findings were confirmed by experiments demonstrating that immunoblot-reactive, but not immunoblot-nonreactive, CU-IgG preparations precipitated the FcepsilonRIalpha from FcepsilonRI- alphagamma-transfected cells. No anti-FcepsilonRIalpha reactivity was observed in IgG fractions from atopic dermatitis (AD) patients (0/15) or healthy control individuals (CO:0/15). As opposed to the selective occurrence of IgG anti-Fc epsilon RI alpha autoantibodies in CU patients, IgG anti-IgE antibodies were detected in all groups investigated (CU: 69%; AD: 73%; CO: 26%). While both types of autoantibodies can exhibit histamine-releasing properties, not all of the autoantibodies proved to be functional in vitro. Our results indicate that the occurrence of IgG anti-FcepsilonRIalpha reactivity defines an autoimmune-mediated subentity of CU and provide a basis for the development of new diagnostic procedures and, perhaps, therapeutic strategies for this disease.