A subset of patients with idiopathic chronic urticaria (CU) has been recently classified as autoimmune on the basis of two main findings: association with thyroid autoimmunity and with anti-IgE and/or anti-IgE receptor antibodies. The association of CU with thyroid autoimmunity has been known since 1983, but its frequency varies in different reports. The objective of the present study was to verify the prevalence of thyroid antibodies (anti-thyroid peroxidase, TPO; thyroglobulin, TG; TSH-receptor, TSH-R) in two distinct series of CU: of known cause (70 cases, group A) and idiopathic (52 cases, group B). Twenty-three patients (M/F:7/16) of group A (33%) and 12 (M/F:4/8) of group B (23%) tested positive for at least one type of thyroid antibody. The difference was not statistically significant. Thyroid disease or altered serum TSH levels (requiring treatment) were present in 39% of group A and 42% of group B seropositive patients. In conclusion, the present study shows that CU, either of known cause or idiopathic, is more common in females than in males and is significantly associated with thyroid autoimmunity. These results were not expected on the assumption that autoimmune phenomena are a specific pattern of idiopathic CU. Thus, screening for thyroid autoimmunity and function is advisable in all patients with CU for the early identification of patients requiring either treatment of underlying thyroid dysfunction or follow-up.