IgG and IgM autoantibodies directed against IgE were determined in 95 serum samples from 67 patients with systemic lupus erythematosus, by an enzyme-linked immunosorbent assay. IgM anti-IgE autoantibodies were found in 18 patients (27%) and IgG anti-IgE autoantibodies were detected in 23 patients (34%). The specificity of the immunoassay was confirmed by the ability to inhibit binding with IgE myeloma, but not other immunoglobulin isotypes and the demonstration that the reactivity was directed to the Fc epsilon fragment of IgE. The IgG fraction of certain sera with anti-IgE activity was capable of inducing histamine release from control basophils and cutaneous mast cells. Clinical associations with the presence of anti-IgE activity were sought by retrospective chart analysis of 61 patients. Significant correlation was found with articular involvement, lymphadenopathy, and anti-DNA antibodies (P less than 0.05). Anti-IgE autoantibodies are observed in a significant number of patients with systemic lupus erythematosus and may contribute to the pathogenesis of the vascular and articular lesions characteristic of this disease.