From a pool of 624 patients with idiopathic chronic urticaria and angioedema, 90 patients had evidence of associated thyroid autoimmunity (TA). Since the number expected by chance alone is 37, given that less than 6% of normal subjects have TA, the association is significant (p less than 0.01; chi-square test). Age and sex distribution was typical of patients with TA. Clinically, most patients suffered relentless and severe urticaria and/or angioedema. With the exception of thyroid function and thyroid antibody tests, other laboratory tests were not rewarding. In most cases, treatment with 1 thyroxine did not improve urticaria or angioedema, but a few patients demonstrated a dramatic response. Awareness of the association resulted in the identification of previously undiagnosed thyroid disease. The authors hypothesize that a subset of idiopathic chronic urticaria and angioedema may be an autoimmune disease.