Backgrounds: The metabolic syndrome is associated with an increased risk of cardiovascular diseases. However, the difference in metabolic abnormalities in patients with acute coronary syndrome (ACS) and no previous diagnosis of DM has not been reported yet. We hypothesize that the disease severity of ACS shows more ominous features with adverse prognosis in patients with metabolic syndrome, especially in young male. Methods: 169 male subjects under 45 years of age who were diagnosed with ACS were enrolled in the study. The mean clinical follow up duration was 18.2짹10.15 months. Metabolic syndrome was defined using a modification of the World Health Organization criteria. The total score of metabolic abnormalities (i.e., abnormal glucose tolerance, hypertension, obesity, hyper-triglyceridemia, and low levels of HDL ; 1 point each) was measured in each group also. Primary end point was defined as re-hospitalization due to cardiovascular event and target vessel revascularization. Results: The prevalence of metabolic syndrome in the study group was 42%. According to the angiographic data, patients with metabolic syndrome showed significantly higher incidence of multi-vessel involvement (p<0.05) and lesion complexity (p<0.001). The overall rate of MACE was significantly higher in patients with metabolic syndrome compared to the control group.( 16.6% vs 7.21% p<0.001). The metabolic syndrome group showed a significantly lower 4 year event free survival survival rate compared to control.[88.8vs71.9, p value<0.05] The cox regression analysis showed metabolic syndrome to be an independent risk factor for adverse event free survival.[ 2.57(1.09-6.49), p-value<0.05] Conclusions: Patients with metabolic syndrome showed significantly higher incidence of multi-vessel involvement and lesion complexity in acute coronary syndrome with adverse prognosis for event free survival. Metabolic syndrome was an independent risk factor for adverse event free survival, which demonstrated the need for early screening and aggressive disease modification regardless of patient age.