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Background: A critical question regarding the association between short-term exposure to ozone and mortality is the extent to which this relationship is confounded by ambient exposure to particles., Objectives: We investigated whether particulate matter < 10 and < 2.5 microm in aerodynamic diameter (PM(10) and PM(2.5)) is a confounder of the ozone and mortality association using data for 98 U.S. urban communities from 1987 to 2000., Methods: We a) estimated correlations between daily ozone and daily PM concentrations stratified by ozone or PM levels; b) included PM as a covariate in time-series models; and c) included PM as a covariate as in d), but within a subset approach considering only days with ozone below a specified value., Results: Analysis was hindered by data availability. In the 93 communities with PM(10) data, only 25.0% of study days had data on both ozone and PM(10). In the 91 communities with PM(2.5) data, only 9.2% of days in the study period had data on ozone and PM(2.5). Neither PM measure was highly correlated with ozone at any level of ozone or PM. National and community-specific effect estimates of the short-term effects of ozone on mortality were robust to inclusion of PM(10) or PM(2.5) in time-series models. The robustness remains even at low ozone levels (< 10 ppb) using a subset approach., Conclusions: Results provide evidence that neither PM(10) nor PM(2.5) is a likely confounder of observed ozone and mortality relationships. Further investigation is needed to investigate potential confounding of the short-term effects of ozone on mortality by PM chemical composition.