Introduction: ST-elevation myocardial infarction (STEMI) is a leading cause of cardiovascular death and thus accounts for a high burden on health care services worldwide. Traditionally, assessment of left ventricular systolic function is concentrated on measurement of left ventricular ejection fraction (LVEF) which is load dependent and sensitive to the preload and after-load. However, myocardial performance index (MPI) demonstrates supremacy over older established indexes. Objective: The purpose of the study is to correlate in-hospital outcome with myocardial performance index (MPI) and left ventricular systolic function in first attack of ST elevated myocardial infarction. Methods: This cross-sectional analytical study was conducted in Dpt. Of Cardiology, Mymensingh Medical College Hospital, Mymensingh, Bangladesh from January to December 2022. Total 120 patients inflicted with first attack of ST elevated myocardial infarction were included considering inclusion and exclusion criteria. The sample population was divided into three groups: Group–I: Patients with mild LV systolic dysfunction (LVEF: 45- 54%), Group–II: Patients with moderate LV systolic dysfunction (LVEF: 35-44%) & Group–III: Patients with severe LV systolic dysfunction (LVEF: <35%). Then In-hospital outcome, LVEF and MPI values were correlated. Results: In this study 120 patients were enrolled. The mean age of the study group was 54.47±11.65, among them male were 105 (87.5%) & female were 15 (12.5%). 81 (54.7%) were hypertensive, 70 (47.3%) were diabetic, 27 (18.2%) having positive family history of CAD, 81 (54.7%) are current smoker, 99 (66.9%) dyslipidaemic & 15 (10.1%) were asthmatic. The mean Troponin-I & NT- Pro BNP levels were 20.57±10.73 & 183.02±29 respectively. The mean LVEF of the groups were: 47.30±3.08, 36.17±1.51 & 25.00±6.05 respectively. The mean MPI of the groups were: 0.32±0.15, 0.45±0.05 & 0.75±0.18 which were statistically significant. Analysis showed that patients with highest level of MPI had severe left ventricular systolic dysfunction (LVEF <35%) with worse in-hospital outcome and vice versa-the patients with the lowest levels of MPI had better systolic function (LVEF ≥ 45%) & in-hospital outcome. Conclusion: The research team was able to conclude that left ventricular ejection fraction and myocardial performance index were significantly correlated with each other & in-hospital outcome; more severe function, more the myocardial performance index with worse in-hospital outcome.