Objectives: To determine the epidemiology of recurrent acute rhinosinusitis (RARS) and to understand direct health care costs attributable to RARS as a potentially underdiagnosed form of chronic rhinosinusitis., Study Design: Retrospective longitudinal analysis of a medical claims database., Setting: Academic medical center., Methods: Medical claims data (2003-2008) from a large payer database were analyzed. Adult patients with RARS (defined as at least 4 acute rhinosinusitis claims each with a filled oral antibiotic prescription in a 12-month period) were extracted. Sinonasal diagnostic procedures, provider visits, and medical costs were determined., Results: A total of 4588 patients were identified (mean age, 43.5 years; 72.1% female) among 13.1 million patients, for a point prevalence of 0.035%, which remained consistent across years. After 1, 2, and 4 years, 2.4%, 5.4%, and 9.2% of patients subsequently received nasal endoscopy and 11.4%, 23.5%, and 39.9% received paranasal sinus computed tomography, respectively. RARS patients averaged 3.8 antibiotic prescriptions, 5.6 other sinus-related prescriptions, and 5.6 provider visits per year. Total direct health care costs related to RARS averaged $1091/patient-year, with oral antibiotic and nasal prescription costs averaging $210 and $452 per year, respectively., Conclusions: RARS may affect approximately 1 in 3000 adults per year. Despite significant direct health care costs of more than $1000/year per individual patient with RARS, nasal endoscopy and computed tomography are not commonly obtained early after potential diagnosis. RARS is likely an underdiagnosed condition warranting further study.