Purpose: To compare the characteristics and prognosis of acute myocardial infarctions (AMIs) that were not the primary reason for hospitalization, and thus not primary discharge diagnosis, to AMIs that were the primary reason for hospitalization. Methods: Primary discharge diagnoses for Reasons for Geographic and Racial Differences in Stroke study participants (black and white men and women age ≥45years) with adjudicated AMIs were categorized as "AMI" or "other". Cox models were used to compare mortality up to 5years post-AMI between primary discharge diagnoses of AMI and other. Results: Of 871 AMIs, primary discharge diagnosis was not AMI in 550 (63%). When primary discharge diagnosis was not AMI, average troponin elevations were smaller and heart failure was more common. Adjusted for participant and hospitalization characteristics, all-cause, coronary heart disease, and cardiovascular disease mortality after AMI were similar between groups (hazard ratios [95% confidence intervals]: 1.08 [0.80-1.47]; 1.29 [0.76-2.18]; and 0.86 [0.58-1.27], respectively). Conclusions: Studies limited to individuals with primary discharge diagnosis of AMI may underestimate the burden of AMI and exclude a group with elevated risk of all-cause, coronary heart disease, and cardiovascular disease mortality.