BACKGROUND: An association between exposure to vehicular traffic in urban areas and the exacerbation of cardiovascular disease has been suggested in previous studies. This study was designed to assess whether exposure to traffic can trigger myocardial infarction. METHODS: We conducted a case-crossover study in which cases of myocardial infarction were identified with the use of data from the Cooperative Health Research in the Region of Augsburg Myocardial Infarction Registry in Augsburg, in southern Germany, for the period from February 1999 to July 2001. There were 691 subjects for whom the date and time of the myocardial infarction were known who had survived for at least 24 hours after the event, completed the registry’s standardized interview, and provided information on factors that may have triggered the myocardial infarction. Data on subjects’activities during the four days preceding the onset of Symptoms were collected with the use of patient diaries. RESULTS: An association was found between exposure to traffic and the onset of a myocardial infarction within one hour afterward(odds ratio, 2.92; 95 percent confidence interval, 2.22 to 3.83; P< 0.001). The time the subjects spent in cars, on public transportation, or on motorcycles or bicycles was consistently linked with an increase in the risk of myocardial infarction. Adjusting for the level of exercise on a bicycle or for getting up in the morning changed the estimated effect of exposure to traffic only slightly(odds ratio for myocardial infarction, 2.73; 95 percent confidence interval, 2.06 to 3.61; P< 0.001). The subject’s use of a car was the most common source of exposure to traffic; nevertheless, there was also an association between time spent on public transportation and the onset of a myocardial infarction one hour later. CONCLUSIONS: Transient exposure to traffic may increase the risk of myocardial infarction in susceptible persons. BACKGROUND: An association between exposure to vehicular traffic in urban areas and the exacerbation of cardiovascular disease has been suggested in previous studies. This study was designed to assess whether exposure to traffic can trigger myocardial infarction. METHODS: We conducted a case-crossover study in which cases of myocardial infarction identified with the use of data from the Cooperative Health Research in the Region of Augsburg Myocardial Infarction Registry in Augsburg, in southern Germany, for the period from February 1999 to July 2001. There were 691 subjects for whom the date and time of the myocardial infarction were known who had survived for at least 24 hours after the event, completed the registry’s standardized interview, and provided information on factors that may have triggered the myocardial infarction. Data on subjects’activities during the four days preceding the onset of Symptoms were collected with the use of patient diaries. RESULTS: An association wa s found between exposure to traffic and the onset of a myocardial infarction within one hour afterward (odds ratio, 2.92; 95 percent confidence interval, 2.22 to 3.83; P <0.001). The time the subjects spent in cars, on public transportation, or on motorcycles or bicycles was consistently linked with an increase in the risk of myocardial infarction. Adjusting for the level of exercise on a bicycle or for getting up in the morning changed the estimated effect of exposure to traffic only slightly (odds ratio for myocardial infarction, 2.73; 95 percent confidence interval, 2.06 to 3.61; P <0.001). The subject’s use of a car was the most common source of exposure to traffic; ne was, there was also an association between time spent on public transportation and the onset of a myocardial infarction one hour later. CONCLUSIONS: Transient exposure to traffic may increase the risk of myocardial infarction in susceptible persons.