Methicillin-resistant Staphylococcus aureus (MRSA) can cause major illness and death and impose serious economic costs on patients and hospitals. Community-associated MRSA (CA-MRSA) is a growing problem in US hospitals, which are already dealing with high levels of hospital-associated MRSA (HA-MRSA), but little is known about how patient age and seasonal differences in the incidence of these 2 forms of MRSA affect the epidemic. By using national data on hospitalizations and antibiotic resistance, we estimated the magnitude and trends in annual S. aureus and MRSA hospitalization rates from 2005-2009 by patient age, infection type, and resistance phenotype (CA-MRSA vs. HA-MRSA). Although no statistically significant increase in the hospitalization rate was seen over the study period, the total number of infections increased. In 2009, there were an estimated 463,017 (95% confidence interval: 441,595, 484,439) MRSA-related hospitalizations at a rate of 11.74 (95% confidence interval: 11.20, 12.28) per 1,000 hospitalizations. We observed significant differences in infection type by age, with HA-MRSA-related hospitalizations being more common in older individuals. We also noted significant seasonality in incidence, particularly in children, with CA-MRSA peaking in the late summer and HA-MRSA peaking in the winter, which may be caused by seasonal shifts in antibiotic prescribing patterns.