Acute otitis externa (AOE) (swimmer's ear) is inflammation of the external auditory canal most often caused by bacterial infection. AOE is characterized by pain, tenderness, redness, and swelling of the external ear canal, and occasionally, purulent exudate. AOE is associated with water exposure (e.g., recreational water activities, bathing, and excessive sweating) and warm, humid environments. Because the overall burden and epidemiology of AOE in the United States have not been well described, data from national ambulatory-care and emergency department (ED) databases were analyzed to characterize the incidence, demographics, and seasonality of AOE and associated health-care costs. The analysis showed that in 2007, an estimated 2.4 million U.S. health-care visits (8.1 visits per 1,000 population) resulted in a diagnosis of AOE. Estimated annual rates of ambulatory-care visits for AOE during 2003-2007 were highest among children aged 5-9 years (18.6) and 10-14 years (15.8); however, 53% of visits occurred among adults aged ≥20 years (5.3). Incidence peaked during summer months, and the regional rate was highest in the South (9.1). Direct health-care costs for nonhospitalized AOE visits total as much as $0.5 billion annually, and ambulatory-care clinicians spend nearly 600,000 hours annually treating AOE. Suggested AOE prevention measures include reducing exposure of the ears to water (e.g., using ear plugs or swim caps and using alcohol-based ear-drying solutions) (3-5). To reduce the national incidence of AOE, additional preventive measures should be investigated, and effective prevention messages should be developed and disseminated.