Background: Acute myocardial infarction (AMI) is a leading cause of mortality worldwide. Left ventricular Ejection Fraction (LVEF) is the most important predictor of mortality after AMI. Thus, this study was aimed to examine the association between smoking cessation and LVEF at one-year follow-up in patients with STEMI.Methods: This study was a part of the Kermanshah STEMI Registry. After applying inclusion criteria, a total of 825 patients were assessed. The data were collected using a standardized case report developed by the European Observational Registry Program (EORP). We compared LVEF measured by an expert cardiologist in people who quit smoking and those who resumed smoking. We used one-to-one propensity score matching (PSM) analysis to minimize the effects of confounding factors on the relationship between LVEF and smoking.Results: At one-year follow-up, 73.45% (606/825) of patients resumed regular smoking, and 26.55% quitted smoking (219/825). The mean of LVEF in those who resumed smoking was higher than those who quit smoking, but the difference was not significant. The results of multivariate logistic regression demonstrated that OR of LVEF decreasement had increased in people who quit smoking (odd ratio (OR) 1.13; 95% CI: 0.98-1.29) compared to those who resumed smoking but not significant, even after applying PSM (OR 1.02; 95% CI: 0.82-1.22).Conclusion: Given the low rate of smoking cessation, physicians are recommended to ask about smoking at each office visit or re-hospitalization and to give a clear and strong smoking cessation recommendation to patients who continue to smoke.