The objective of this research was to evaluate the effect of particulate matter air pollution, including emissions from diesel generators, on visits to emergency departments for asthma. Daily asthma case data from participating hospitals in the greater Tacoma, Washington area were obtained. Daily asthma emergency room visit data were available from six Tacoma hospitals from January 3, 1998 to May 30, 2002. Only emergency visits where the primary discharge diagnosis was asthma were included in the analysis. Air pollution, daily temperature and relative humidity data were obtained from the Puget Sound Clean Air Agency. An association between daily PM2.5 and emergency department (ED) visits for asthma at lag days 2 and 3 was observed. The relative risk for lag day 2 was 1.04 (95% confidence interval[CI]: 1.01, 1.07) and for lag day 3 was 1.03 (1.0, 1.06). A significant association between ED visits for asthma and increased use of diesel generators was not detected. The use of low-sulfur diesel oil may have mitigated potential adverse health effects. These data indicate that air pollution in a medium-sized coastal city may be sufficient to have a public health impact on asthma.