Background: Previous studies defining the clinical features of patients with chronic idiopathic urticaria (CIU) were performed before the identification of functional autoantibodies against FcϵRI and/or IgE, now known to be present in approximately 30% of patients with CIU. Objective: Our purpose was to determine whether there are differences between patients with and those without autoantibodies in the clinical features or severity of CIU. Methods: The clinical features of 107 patients with CIU were evaluated prospectively. Patients were identified as having functional autoantibodies on the basis of the serum-evoked histamine release in vitro from the basophils of 2 healthy donors. Results: Patients with autoantibodies (31%) had more wheals (P = .005), a wider distribution of wheals (P = .009), higher itch scores for the most severe episodes of itching (P = .002), more systemic symptoms (P = .03), and lower serum IgE levels (P < .0005) than patients without autoantibodies. Conclusion: The presence of autoantibodies indicates a subset of patients with more severe CIU. (J Am Acad Dermatol 1999;40:443-50.)