Previous studies indicate that 30-50% of chronic urticaria patients have an autoimmune etiology. Clinical diagnosis of autoimmune urticaria is supported with the autologous serum skin test. The purpose of this study was to compare two laboratory tests for measurement of IgG autoantibodies to IgE or IgE receptors and compare the results with the autologous serum and plasma skin tests. We performed skin tests and two functional in vitro tests, basophil histamine release, and CD63 up-regulation to detect autoantibodies relevant to autoimmune urticaria. Both sera and citrated plasma were evaluated in the autologous skin test and histamine release assay. Thyroid autoantibodies were also measured. Basophils were incubated with patient plasma, sera, buffer, or anti-IgE. The cells were analyzed for CD63 expression and the supernatants were recovered for histamine analysis. There was high correlation between CD63 up-regulation and histamine release assays, but histamine release was more sensitive. There was a high concordance between sera and citrated plasma for the skin test. Sera from chronic urticaria patients produced higher mean histamine release (23%) compared with citrated plasma (12%). Thirty-one percent of patients positive in the histamine release assay were also positive for thyroid autoantibodies. This compares with 12% who were negative in the histamine release assay. These data show that in vitro basophil histamine release can be used to measure antibodies to FceRI, FceRII/CD23, or IgE and identify patients with autoimmune urticaria.