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Findings from previous cohort studies suggest a positive association between traffic-related air pollution and cardiovascular mortality. However, few studies have assessed intraurban variation in traffic-related pollution or evaluated cardiovascular effects at lower levels of pollution that are typically seen in Canadian cities.We conducted a cohort study of traffic-related air pollution and cardiovascular mortality among adults who lived in three cities in Ontario, Canada. Study members of the cohort were a random sample from the federal family income tax database, comprising 205,440 adults age 35–85 years, who lived in Toronto, Hamilton, or Windsor between 1982 and 1986. Follow-up ended on 31 December 2004. Mortality from cardiovascular and cerebrovascular diseases was ascertained using the Canadian Mortality Database. We estimated time-dependent concentrations of ambient nitrogen dioxide (NO2) from land-use regression models and assigned exposures to residences of subjects. Rate ratios (RRs) were estimated from Cox proportional hazard model adjusted for individual risk factors and selected contextual covariables. We adjusted indirectly for smoking and obesity.The spatial distributions of NO2 did not change appreciably over the follow-up period. Cumulative exposure to NO2 was associated with a 12% increase in mortality from cardiovascular disease for each increase of 5 parts per billion of NO2 (95% confidence interval [CI] = 7%–17%) and a 15% increase (8%–21%) in mortality from ischemic heart disease. Risks of cardiovascular mortality were also increased with shorter term exposures, but the RRs were somewhat smaller. No association was found for cerebrovascular mortality (RR = 0.99 [95% CI = 0.91–1.08]).Traffic-related air pollution at relatively low concentrations in Ontario was associated with increased mortality from cardiovascular disease.