OBJECTIVES Acute myocardial infarction (AMI) is a major cause of mortality and disability worldwide. AMI occurs when blood flows irregularly into heart and thus heart muscle is injured due to insufficient constant oxygen received. It will cause severe complications or co-morbidities if the condition is lasting. Hence, we hypothesized the atrial fibrillation (AF) is an independent risk factor to the major severe cardiovascular events (MACE) after AMI occurred.  METHODS The AMI patient is defined by the patient treated in the emergency room at the beginning of the illness. Frequencies and costs of AMI data were extracted from the National Health Insurance Research Database for this observational retrospective cohort study between 2007 and 2012 in Taiwan. ICD-9-CM 410 was used to extract the AMI patients. Fisher’s exact test and categorical data analytic method were utilized to assess the AF as a risk factor to MACE in the AMI patients. RESULTS We mainly focused on the AMI adults without any prior MACE occurred. As a result, there were 3,452 AMI who can be divided into 2 groups: 2,939 AF patients and 513 non-AF patients. The average medical cost was USD$ 3142.8 and the mean LoS was 10.4 days with st.d. 29.0 days. There were 1,791 MACE identified among the AF patients (60.9%), while there were 251 MACE among the non-AF patients (48.9%). In consequence, the AMI with AF resulted more MACE than those in non-AF (RR = 1.63, C.I. = (1.35, 1.97), p <0.0001). The difference of cost was not significant between both groups. The mean LoS in AF was 16.6 days, which was significantly smaller than that in non-AF (19.1 days). The difference was probably due to higher fatalities in AF. CONCLUSIONS This study has demonstrated that AF risk is associated with MACE in patients after AMI occurred.