Substantial evidence supports an association between ambient air pollution, especially particulate matter (PM*) and ozone (O3), and acute cardiovascular and respiratory morbidity. There is increasing interest in accountability research to evaluate whether actions taken to reduce air pollution will result in reduced morbidity. This study capitalized on a unique opportunity to evaluate the impact of a local, short-term intervention effort to reduce traffic in Atlanta during the 1996 Summer Olympic Games (July 19-August 4). Air pollutant concentrations both inside and outside of Atlanta were examined during the Olympic period and surrounding periods. Emergency department (ED) visits were examined to evaluate changes in usage patterns. ED visits for respiratory and cardiovascular conditions were examined in relation to the Olympic period using Poisson time-series analysis with adjustment for time trends and meteorologic conditions. O3 concentrations were approximately 30% lower during the Olympic Games compared with the four weeks before and after the Olympic Games (baseline periods); however, we observed similar reductions in O3 concentrations in several other cities in the Southeastern United States. We observed little or no evidence of reduced ED visits during the Olympic Games; the estimates were sensitive to choice of analytic model and to method of adjusting for temporal trends. The meteorologic conditions during the Olympic Games, along with the reductions in O3 observed in various cities not impacted by the Olympic Games, suggest that both meteorologic conditions-and reduced traffic may have played a role in the observed reduction in O3 concentration in Atlanta. Additionally, it is likely that this particular intervention strategy would not be sustainable as a pollution-reduction strategy. This study demonstrates some limitations of conducting retrospective accountability research.