Objective: To examine the relationship between ambulance diversions and the incidence of myocardial infarction deaths in New York City. Methods: We obtained data for 1999 and 2000 on all 9,743 deaths due to myocardial infarction in New York City, as well as periods of diversion status for 58 New York City area hospitals operating under a central ambulance dispatch by the New York City Fire Department. Negative binomial regressions were used to model the percentage increase in myocardial infarction deaths associated with diversion status. Results: On average, 2.67 deaths from myocardial infarction occurred in each New York City borough per day. On the seven borough-days when more than 20 percent of a borough's available emergency department time was spent on diversion, there was a 47 percent (95 confidence interval, 27 to 67 percent) increase in borough-wide deaths from myocardial infarction, or 8.78 additional deaths over the two-year study period. On the 46 borough-days in which at least 25 percent of a borough's emergency departments were simultaneously on diversion, there was a 17 percent (95 percent confidence interval, 2 to 31 percent) increase in myocardial infarction mortality in that borough, or 20.88 additional deaths over the study period. In contrast, there was no association between low levels of ambulance diversion and deaths from myocardial infarctions. Conclusions: In New York City in 1999/2000, both high levels of total ambulance diversion hours as well as high levels of simultaneous hospital diversion were associated with increased deaths from myocardial infarction.