RESEARCH resistance proportion, significant differences were seen by geographic region. S. aureus discharge diagnoses were sig- nificantly higher for the South than the Northeast, while for methicillin resistance proportion, the Northeast, Midwest, and South were significantly higher than the West (p<0.05 for all comparisons). The South had the highest MRSA hospitalization rate, reflecting both the S. aureus rate and methicillin resistance proportion, which was significantly higher than the MRSA rate estimated for the West (South vs. West, relative risk 1.57, 95% confi- dence interval 1.29–1.91). Most S. aureus–related discharge diagnoses occurred in patients ≥65 years of age. When S. aureus diagnoses by rate were examined, a bimodal distribution was seen, with highest rates occurring in children and the elderly (Table 3). Patients ≤14 and 15–44 years of age had higher MRSA hospitalization rates compared with patients 45–64 and ≥65 years of age (p<0.01). Overall, the MRSA rate increased with patient age (p<0.05 for trend). Discussion Infectious diseases cause many hospitalizations each year in the United States; these diseases include syndromes commonly associated with S. aureus. In 1994, the rate of hospitalization for infectious disease was 15 per 1,000 US population, with a total of 4 million hospitalizations, including 1,480,000 pneumonias, 335,000 skin infections, and 302,000 septicemias; yearly rates for these disease syndromes were similar from 1999 to 2000 (12–14). Gram-positive organisms are an increasingly recognized cause of systemic infection, including sepsis (12,15). More than half of all sepsis cases are estimated to be caused by gram-positive organisms, including S. aureus (16). In the Calgary Health Region in Canada, the annual incidence of invasive S. aureus infection was estimated to be 28.4 cases per 100,000 population from 1999 to 2000, which is com- parable with the rate of invasive pneumococcal disease and exceeds the rate of invasive streptococcal infection (17). Drug resistance in S. aureus, including the emergence of MRSA in healthcare and community settings, is an increasingly reported event that makes treating serious infection difficult. Extrapolating from our estimates and those of Simonsen et al. (12), a rate of ≈47 diagnoses per 100,000 population, making up 3% of all infectious dis- ease hospitalizations, were associated with laboratory-con- firmed MRSA infection from 1999 to 2000, and ≈10% of septicemias were caused by MRSA. Although the burden of MRSA infection has not been systematically estimated nationally, past estimates have been based on single-center or selected population-based studies in the United States. Based on ICD-9-CM data from the New York City metropolitan area, an estimated 1.0% of hospital discharges are associated with S. aureus infection, and 0.21% of discharges are estimated to be associated with MRSA (18). In 1995, based on extrapola- tion of hospital discharge data from NHDS and nosocomi- al infection data from the NNIS System, an estimated 206,504 S. aureus infections (0.58% of admissions) and 70,270 MRSA infections (0.20% of admissions) were acquired in the healthcare setting (Centers for Disease 870 Emerging Infectious Diseases • www.cdc.gov/eid • Vol. 11, No. 6, June 2005