The first analysis of long-term exposures to air pollution and risk of mortality using the American Cancer Society Cancer Prevention Study II (ACS CPS-II) cohort was published in 1995.1 Subsequently, extensive independent reanalysis2 and multiple extended analyses3-7 were conducted. These studies have consistently demonstrated that exposure to fine particulate matter air pollution (PM2.5) is associated with increased risk of mortality, especially cardiopulmonary or cardiovascular disease mortality. A recent analysis by Enstrom, based on early data from the ACS CPS-II cohort, reports no significant relationship between PM2.5 and total mortality.8 The author asserts that the original analyses, reanalyses, and the extended analyses found positive PM2.5–mortality relationships because of selective use of CPS-II and PM2.5 data.