rhis issue of EPIDEMIOLOGY contains 3 meta-analyses of the extensive data relating ambient ozone levels to daily mortality.3 When I was a student at Cambridge, my tutor used to throw things across the table at us if we did not always mention the most important fact first; so for those whose attention span is short, all 3 studies report a small but substantial association between ozone levels and total mortality. These studies were commissioned by the same agency, but the authors were free to carry out the analysis as they saw fit, and all 3 differ. One set of authors' used data from 14 U.S. cities, 13 Canadian cities, and 21 European cities, and excluded data from the National Morbidity and Mortality Air Pollution Study (NMMAPS) and from Mexico City. The second2 used the data from the NMMAPS study of 95 cities together with European studies for a total input of 144 datasets. (These authors had already published an analysis of the NMMAPS data alone.4) The third3 was more restricted, using data from 7 U.S. cities plus other worldwide data for various parts of the analysis. Another difference is that one paper1 used data on the prevalence of air conditioning in both the United States and Canada. Bayesian hierarchical models were used in the analyses. PM interaction with ozone was generally found to be unimportant. All 3 studies noted that the response function was higher in summer (when ozone levels are higher) than in winter, which means that if the data are not stratified by season, the overall response outcome is likely to be diminished. Other notable factors were that the prevalence of air conditioning affected the outcome1; the NMMAPS data alone yielded lower response outcomes than most other analyses, and there was generally satisfactory concordance between U.S. and European data. Levy et all found that total mortality increased by 0.86% per 10 ppb in summer. Bell et a12 found an increase of 0.83% per 10 ppb in total mortality overall, and agreed that U.S. and non-U.S. data were similar. Ito et a13 provided a detailed seasonal breakdown and showed that the main effect occurred in the warm season. In their analysis for a single pollutant model, these authors plotted data from 8 U.S. regions, 8 European cities, 2 Australian cities, plus Mexico City, Sao Paulo, Santiago, and 2 regions of South Korea (See Fig. 1 of Ito et al3). Negative data points were noted for 5 cities, and all the rest were positive. The highest was for Brisbane, Australia, at approximately a 3.5% increase in mortality per 10 ppb for the 24-hour average ozone. Reviewing all the data, I would regard the value of 0.86% change in mortality per 10 ppb as a minimal figure, because inclusion of data from Brisbane and Mexico City would increase this significantly. A recent report of European data5 summarized data from 23 regions with mortality data over a 3-year period. The authors reported no association between ozone and mortality during the winter months but an association in summer, with a mean increase of 0.33% in total mortality, 0.45% in cardiovascular deaths, and 1.13% in respiratory deaths