To investigate acute, irreversible effects of exposure to ozone and other air pollutants, the authors examined daily death counts in relation to air pollution levels in Mexico City during 1990–1992. When considered singly in Poisson regression models accounting for penodic effects, the rate ratio for total mortality associated with a 100-ppb increment in 1-hour maximum ozone concentration was 1 024 (95% confidence interval (Cl) 1.011–1.039). Measures of average ozone concentration were somewhat more strongly related to mortality. The rate ratio was 1.024 (95% Cl 0.984–1.062) per 100 ppb for sulfur dioxide and 1 050 (95% Cl 1.030–1.067) per 100 μg/m3 for total suspended particulates. However, when all three pollutants were considered simultaneously, only total suspended particulates remained associated with mortality, indicating excess mortality of 6% per 100 μg/m3 (rate ratio = 1.058, 95% Cl 1.033–1.083), consistent with observations in other cities in the United States and Europe. The authors found no independent effect of ozone, but it is difficult to attnbute observed effects to a single pollutant in light of the complexity and variability of the mixture to which people are exposed. Nevertheless, particulate matter may be a useful indicator of the nsk associated with ambient air pollution. Am J Epidemiol 1997; 145: 258-68