RATIONALE: There are unexplained geographical and seasonal differences in the short-term effects of fine particulate matter (PM(2.5)) on human health. The hypothesis has been advanced to include the possibility that such differences might be due to variations in the PM(2.5) chemical composition, but evidence supporting this hypothesis is lacking. OBJECTIVES: To examine whether variation in the relative risks (RR) of hospitalization associated with ambient exposure to PM(2.5) total mass reflects differences in PM(2.5) chemical composition. METHODS: We linked two national datasets by county and by season: (1) long-term average concentrations of PM(2.5) chemical components for 2000-2005 and (2) RRs of cardiovascular and respiratory hospitalizations for persons 65 years or older associated with a 10-microg/m(3) increase in PM(2.5) total mass on the same day for 106 U.S. counties for 1999 through 2005. MEASUREMENTS AND MAIN RESULTS: We found a positive and statistically significant association between county-specific estimates of the short-term effects of PM(2.5) on cardiovascular and respiratory hospitalizations and county-specific levels of vanadium, elemental carbon, or nickel PM(2.5) content. CONCLUSIONS: Communities with higher PM(2.5) content of nickel, vanadium, and elemental carbon and/or their related sources were found to have higher risk of hospitalizations associated with short-term exposure to PM(2.5).