RESEARCH Methicillin-resistant– Staphylococcus aureus Hospitalizations, United States Matthew J. Kuehnert,* Holly A. Hill,* Benjamin A. Kupronis,* Jerome I. Tokars,* Steven L. Solomon,* and Daniel B. Jernigan* Methicillin-resistant Staphylococcus aureus (MRSA) is increasingly a cause of nosocomial and community-onset infection with unknown national scope and magnitude. We used the National Hospital Discharge Survey to calculate the number of US hospital discharges listing S. aureus–spe- cific diagnoses, defined as those having at least 1 International Classification of Diseases (ICD)-9 code specif- ic for S. aureus infection. The number of hospital discharges listing S. aureus–specific diagnoses was multiplied by the proportion of methicillin resistance for each corresponding infection site to determine the number of MRSA infections. From 1999 to 2000, an estimated 125,969 hospitalizations with a diagnosis of MRSA infection occurred annually, including 31,440 for septicemia, 29,823 for pneumonia, and 64,706 for other infections, accounting for 3.95 per 1,000 hospital discharges. The method used in our analysis may provide a simple way to assess trends of the magnitude of MRSA infection nationally. Sboth healthcare and community settings. It is one of the taphylococcus aureus is a major cause of infection in most common causes of healthcare-associated infections reported to the National Nosocomial Infections Surveillance (NNIS) System, including ventilator-associ- ated pneumonia, surgical site infection, and catheter-asso- ciated bloodstream infection (1). S. aureus is also a frequent cause of community-associated infections, partic- ularly skin and soft tissue infections. Although most com- munity-onset infections are treated in the outpatient setting, some invasive infections, including bacteremia, septic arthritis, toxic shock syndrome, osteomyelitis, and endocarditis, have devastating complications and may require hospitalization (2). *Centers for Disease Control and Prevention, Atlanta, Georgia, USA Antimicrobial resistance in S. aureus emerged soon after penicillin came into common use in the 1940s. During the next 2 decades, resistance of this pathogen to penicillin became widespread, followed by increasing resistance to the new semisynthetic penicillinase-resistant antimicrobial drugs (e.g., methicillin, oxacillin, nafcillin) (3). In the last 20 years, methicillin-resistant S. aureus (MRSA) has spread throughout the world in healthcare set- tings, leading to an increased reliance on vancomycin for empiric treatment (4). Recently, S. aureus resistance to vancomycin, the last commonly used antimicrobial drug to which this organism was considered uniformly suscepti- ble, has emerged (5). In addition, serious MRSA infection has been increasingly reported in persons without identi- fied predisposing risk, including recent healthcare expo- sure (6). MRSA infections are thought to cause substantial ill- ness and contribute to healthcare costs in the United States. However, published estimates vary widely and have been based on single-center or local data with limited applica- bility (4,7). Accurate estimates of the incidence of MRSA infection are essential to determine effects on health and healthcare expenditures. Since most patients with serious MRSA infections are hospitalized, we focused our esti- mate on hospitalized patients. Methods The incidence of S. aureus infection was estimated from the number of hospitalizations with S. aureus–related discharge diagnoses in a national surveillance database. We used 1999 and 2000 public-use data from the National Hospital Discharge Survey (NHDS) to calculate the num- ber of hospital discharges with at least 1 S. aureus–related discharge diagnosis. All acute-care hospitalizations, except infants whose hospital stay began at their own birth, were 868 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 6, June 2005