This article provides an overview of previous analysis and reanalysis of the American Cancer Society (ACS) cohort, along with an indication of current ongoing analyses of the cohort with additional follow-up information through to 2000. Results of the first analysis conducted by Pope et al. (1995) Pope, C. A. III , Thun, M. J. , Namboodiri, M. M. , Dockery, D. W. , Evans, J. S. , Speizer, F. E. and Heath, C. W. 1995. Particulate air pollution as a predictor of mortality in a prospective study of US adults. Am. J. Resp. Crit. Care, 151: 669–674. [CSA] [PubMed], [Web of Science ®] [Google Scholar] showed that higher average sulfate levels were associated with increased mortality, particularly from cardiopulmonary disease. A reanalysis of the ACS cohort, undertaken by Krewski et al. (2000) Krewski, D. , Burnett, R. T. , Goldberg, M. S. , Hoover, K. , Siemiatycki, J. , Jerrett, M. , Abrahamowicz, M. and White, W. H. 2000. Reanalysis of the Harvard Six Cities Study and the American Cancer Society study of particulate air pollution and mortality, Cambridge, MA: Health Effects Institute. [Google Scholar] , found the original risk estimates for fine-particle and sulfate air pollution to be highly robust against alternative statistical techniques and spatial modeling approaches. A detailed investigation of covariate effects found a significant modifying effect of education with risk of mortality associated with fine particles declining with increasing educational attainment. Pope et al. (2002) Pope, C. A. III , Burnett, R. T. , Thun, M. J. , Calle, E. E. , Krewski, D. , Ito, K. and Thurston, G. D. 2002. Lung cancer, cardiopulmonary mortality, and long-term exposure to fine particulate air pollution. J. Am. Med. Assoc., 287: 1132–1141. [CROSSREF] [Crossref], [PubMed], [Web of Science ®] [Google Scholar] subsequently reported results of a subsequent study using an additional 10 yr of follow-up of the ACS cohort. This updated analysis included gaseous copollutant and new fine-particle measurements, more comprehensive information on occupational exposures, dietary variables, and the most recent developments in statistical modeling integrating random effects and nonparametric spatial smoothing into the Cox proportional hazards model. Robust associations between ambient fine particulate air pollution and elevated risks of cardiopulmonary and lung cancer mortality were clearly evident, providing the strongest evidence to date that long-term exposure to fine particles is an important health risk. Current ongoing analysis using the extended follow-up information will explore the role of ecologic, economic, and, demographic covariates in the particulate air pollution and mortality association. This analysis will also provide insight into the role of spatial autocorrelation at multiple geographic scales, and whether critical instances in time of exposure to fine particles influence the risk of mortality from cardiopulmonary and lung cancer. Information on the influence of covariates at multiple scales and of critical exposure time windows can assist policymakers in establishing timelines for regulatory interventions that maximize population health benefits.