Background: Extreme temperatures have been associated with increased mortality worldwide. The extent to which air pollutants may confound or modify this association remains unclear. Methods: We examined the association between mean apparent temperature and total mortality in 9 cities across the United States during the warm season (May to September) from 1999 to 2002. We applied case-crossover and time-series analyses, adjusting for day of the week and season in time-series analysis. City-specific estimates were then combined using a meta-analysis. A total of 213,438 deaths for all causes occurred in these cities during the study period. Results: We found that mortality increased with apparent temperature. A 5.5 degrees C (10 degrees F)increase in apparent temperature was associated with an increase in mortality of 1.8% (95% confidence interval Euro Surveillance (Bulletin Europeen Sur Les Maladies Transmissibles; European Communicable Disease Bulletin) 1.09% to 2.5%) when using case-crossover analysis and with an increase of 2.7% (2.0% to 3.5%) using the time-series analysis. Conclusions: This study provides evidence of increased mortality due to elevated apparent temperature exposure, with no confounding or effect modification due to air pollution.