Obesity is an important risk factor for the development of dyslipidemia, diabetes mellitus, hypertension, coronary artery disease, ventricular dysfunction, congestive heart failure (HF), stroke, and cardiac arrhythmias. This meta-analysis brings comprehensive evaluation about still controversial association between the body mass index (BMI) and the outcomes of acute coronary syndrome. PubMed/ScienceDirect databases were systematically searched for studies with baseline parameters, primary (HF, cardiogenic shock, cardiac arrest, reinfarction, stroke, death, total in-hospital complications) and secondary outcomes (reinfarction, stroke, death, total major adverse cardiovascular events MACE) in relation to BMI strictly classified into four groups (underweight <18.5 kg/m2, normal weight 18.5-24.9 kg/m2, overweight 25.0-29.9 kg/m2, and obese ≥30.0 kg/m2, grouped into mildly obese 30.0-34.9 kg/m2 and severely obese ≥35.0 kg/m2). We included 24 studies, with 585,919 participants (55.5% males), aged 66.8 years. Underweight was negatively associated with hypertension, hyperlipidemia and diabetes, and positively with primary outcomes {HF (odds ratio OR = 1.37, confidence interval CI 1.15-1.63), cardiogenic shock (OR = 1.43, CI 1.04-1.98), stroke (OR = 1.21, CI 1.05-1.40), overall death (OR = 1.64, CI 1.20-2.26), total in-hospital complications (OR = 1.39, CI 1.24-1.56)} and secondary outcomes during 34-month follow-up {cardiovascular/overall death (OR = 3.78, CI 1.69-8.49/OR = 2.82, CI 2.29-3.49), respectively, total MACE (OR = 2.77, CI 2.30-3.34)} (for all p < 0.05). Obesity had positive association with hypertension, hyperlipidemia, diabetes and smoking, and negative with primary outcomes {reinfarction (OR = 0.83, CI 0.76-0.91), stroke (OR = 0.67, CI 0.54-0.85), overall death (OR = 0.55, CI 0.49-0.63), total in-hospital complications (OR = 0.81, CI 0.70-0.93)} and secondary outcomes {cardiovascular/overall death (OR = 0.77, CI 0.66-0.88/OR = 0.62, CI 0.53-0.72), respectively, total MACE (OR = 0.63, CI 0.60-0.77)} (for all p < 0.05). This negative association with several primary outcomes (cardiogenic shock, overall death, total in-hospital complications) and secondary outcomes (cardiovascular/overall death, total MACE) was more pronounced in mild obesity (p < 0.05). These results give an "obesity paradox" with a bimodal pattern (slightly U-shaped). Obesity is positively associated with traditional cardiovascular risk factors and negatively with primary and secondary outcomes, which confirms the persistence of overall "obesity paradox."