Background: obesity is defined as body mass index (BMI) ≥30 kg/m 2 . Obesity is currently the most common metabolic disease. It is associated with many metabolic and cardiovascular diseases, thereby contributing to increased morbidity and mortality. Recent studies have described a better outcome of obese patients in the setting of acute ST segment elevation myocardial infarction (STEMI).Objectives: to study the impact of BMI on the presentation and in-hospital outcome of acute STEMI.Methods: a total of 140 STEMI patients were enrolled in our study, those patients were treated mainly by primary PCI. Patients were classified into two groups according to BMI; obese group with BMI ≥ 30kg/m 2 and non obese group with BMI < 30kg/m 2 . Both groups were compared regarding presentation and inhospital outcome.Results: seventy six patients (54.3%) were obese; they were significantly younger, more hypertensive, diabetic and dyslipidemic. Obese patients presented earlier (P= 0.001) and in better Killip class than non obese (P= 0.017). 37 non obese patients (57.8%) had post MI complications compared to 18 obese patients (23.7%) (P= 0.000), mainly heart failure, tachyarrhythmias, heart block and post MI angina. Patients with low BMI had complications 4 times more than patients with high BMI (Odds ratio = 4.416, 95% C.I. from 2.138 to 9.118, P< 0.001).Conclusion: the current study highlights an apparent obesity paradox showing that the non-obese STEMI patients were found to have complications 4 times more than the obese patients. Caution should be taken to prevent confusion between risk marker and risk factor.