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Extreme temperatures have been associated with increased mortality worldwide. The extent to which air pollutants may confound or modify this association remains unclear.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.We found that mortality increased with apparent temperature. A 5.5°C (10°F) increase in apparent temperature was associated with an increase in mortality of 1.8% (95% confidence interval = 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.This study provides evidence of increased mortality due to elevated apparent temperature exposure, with no confounding or effect modification due to air pollution.