Background: Studies on the prognosis of ST ELEVATION myocardial infarction (STEMI) versus non-ST ELEVATION myocardial infarction (non-STEMI) have shown different results. The present study was designed to compare the early outcome and left ventricular systolic function of patients with ST and non-ST ELEVATION myocardial infarction. Methods: The patients' information was derived from 10,065 consecutive patients hospitalized in Tehran Heart Center with acute MI (2007 patients with STEMI and 8058 with non-STEMI). The baseline clinical characteristics, post-MI complications, left ventricular systolic functions, and 30-day mortality rates were compared. Results: A history of current cigarette smoking, opium addiction, and brain stroke was more frequent in the STEMI patients, whereas hyperlipidemia, hypertension, and obesity were found more in the non-STEMI group. Ejection fraction was higher in the non-STEMI patients than that in the STEMI group, and anterior wall infarction was detected more frequently in the STEMI cases. A history of coronary artery bypass grafting and also percutaneous coronary intervention was observed more in the non-STEMI group. Amongst the in-hospital complications, ventricular arrhythmias (1.4 vs. 0.5, P<0.001) and pulmonary edema (0.4 vs. 0.1, P=0.002) were more prevalent in the STEMI cases. The 30-day mortality rate in the STEMI group was higher than that in the non-STEMI group (5.5 vs. 2.4, P<0.001). Early mortality in both groups was dependant on advanced age, diabetes mellitus, post-MI bradycardia, and atrioventricular block. Also, female gender and pulmonary edema in the STEMI group and family history of MI in the non-STEMI patients could predict 30-day mortality. Conclusion: There were several differences in the baseline characteristics and early outcome between the two types of STEMI and non-STEMI. The 30-day mortality rate was higher in the STEMI group than that in the non-STEMI group.