Evaluation of the public health impact of air quality regulations, referred to as accountability research, is increasingly viewed as a necessary component of responsible governmental policy interventions. The authors present an example of accountability assessment based on evaluating change in the short-term effect of airborne particles over a period of increasingly stringent regulation that might have changed the chemical composition and toxicity of these particles. They used updated data and methods of the National Morbidity Mortality Air Pollution Study to estimate national average relative rates of the effects of particulate matter <or=10 microm in aerodynamic diameter on all-cause, cardiovascular, and respiratory mortality and on other-cause mortality for 1987-2000. They estimated national average relative rates of the effects of particulate matter <or=2.5 microm in aerodynamic diameter on all-cause mortality for 1999-2000. The authors found strong evidence that lag 1 exposures to particulate matter <or=10 microm and <or=2.5 microm in aerodynamic diameter continue to be associated with increased mortality. They also found a weak indication that the lag 1 effects of particulate matter <or=10 microm in aerodynamic diameter on mortality declined during 1987-2000 and that this decline occurred mostly in the eastern United States. The methodology presented can be used to track the health effects of air pollution routinely on regional and national scales.