The association between daily mortality and respirable particulate pollution (PM10) in Utah County was assessed from April 1985 through December 1989. Poisson regression analysis was used to regress daily death counts on PM10 pollution levels, controlling for variability in the weather. A significant positive association between nonaccidental mortality and PM10 pollution was observed. The strongest association was with 5-d moving average PM10 levels, including the concurrent day and the preceding 4 d. An increase in 5-d moving average PM10 levels, equal to 100 micrograms/m3, was associated with an estimated increase in deaths per day equal to 16%. The association with mortality and PM10 was largest for respiratory disease deaths, next largest for cardiovascular deaths, and smallest for all other deaths. Mean PM10 concentrations during the study period equaled 47 micrograms/m3. The maximum 24-h and 5-d moving average PM10 levels equaled 365 and 297 micrograms/m3, respectively. Relatively low levels of sulfur dioxide, aerosol acidity, and ozone suggested an independent association between mortality and PM10. The relative risk of death increased monotonically with PM10, and the relationship was observed at PM10 levels that were well below the current National Ambient Air Quality Standard of 150 micrograms/m3.