Adverse health effects of particulate matter <10 μm in aerodynamic diameter (PM10) and high temperatures are well known, but the extent of their interaction on mortality is less clear. This paper describes effect modification of temperature in the PM10–mortality association and tests the hypothesis that higher PM10 effects in summer are due to enhanced exposure to particles. All deaths of residents of nine Italian cities between 1997 and 2004 were selected. The case-crossover approach was adopted to estimate the effect of PM10 on mortality by season and temperature level. Three strata of temperature corresponding to low, medium, and high “ventilation” were identified, and the interaction between PM10 and temperature within each stratum was examined. Season and temperature levels strongly modified the PM10–mortality association: for a 10-μg/m3 variation in PM10, a 2.54% increase in risk of death in summer (95% confidence interval: 1.31, 3.78) compared with 0.20% (95% confidence interval: −0.08, 0.49) in winter. Analysis of the interaction between PM10 and temperature within temperature strata resulted in positive but, in most cases, nonstatistically significant coefficients. The authors found much higher PM10 effects on mortality during warmer days. The hypothesis that such an effect is attributable to enhanced exposure to particles in summer could not be rejected.