Background: Seasonal and regional differences have been reported for the increase in short-term mortality associated with a given increase in the concentration of outdoor particulate matter with an aerodynamic diameter smaller than 10μm (PMââ mortality coefficient). Some of this difference may be because of seasonal and regional differences in indoor exposure to PMââ of outdoor origin. Methods: From a previous study, we obtained PMââ mortality coefficients for each season in seven U.S. regions. We then estimated the change in the sum of indoor and outdoor PMââ exposure per unit change in outdoor PMââ exposure (PMââ exposure coefficient) for each season in each region. This was originally accomplished by estimating PMââ exposure coefficients for 19 cities within the regions for which we had modeled building infiltration rates. We subsequently expanded the analysis to include 64 additional cities with less wellcharacterized building infiltration rates. Results: The correlation (r = 0.71 [95% confidence interval = 0.46 to 0.86]) between PMââ mortality coefficients and PMââ exposure coefficients (28 data pairs; four seasons in each of seven regions) was strong using exposure coefficients derived from the originally targeted 19 National Morbidity, Mortality, and Air Pollutions Study cities within the regions. The correlation remained strong (r = 0.67 [0.40 to 0.84]) when PMââ exposure coefficients were derived using 83 cities within the regions (the original 19 plus the additional 64). Conclusions: Seasonal and regional differences in PM 10 mortality coefficients appear to partially reflect seasonal and regional