OBJECTIVES: Chronic idiopathic/spontaneous urticaria (CIU) is a skin condition characterized by the presence of hives and angioedema lasting ≥6 weeks with no known cause. There is no specific ICD-9 code for CIU. This study aims to validate an ICD-9 based algorithm to identify patients with CIU to allow for epidemiologic research using administrative claims databases METHODS: Retrospective review of medical and billing records in 4 geographically-dispersed US allergy practices. Patients were eligible for inclusion if they met the either of these criteria: 1) ≥2 outpatient visits with a diagnosis of idiopathic urticaria (ICD-9 708.1), other specified urticaria (708.8), or unspecified urticaria (708.9) ≥6 weeks apart, OR 2) ≥1 outpatient diagnosis of 708.1, 708.8 or 708.9 and 1 diagnosis of angioedema (995.1) ≥6 weeks apart. A random sample of patients known to have CIU were also included. Chart review was performed to collect information on ICD-9 codes, prescription medications, and clinical urticarial diagnoses abstracted from records of a random sample of eligible patients. Sensitivity and positive predictive value (PPV) were calculated.  The study received central IRB approval. RESULTS: There were