BACKGROUND: There are a few literature data on the correlation between metabolic syndrome (MetS) and coronary disease among Iranian population. This study aimed to find relationship between MetS and severity of coronary artery disease (CAD) in presence of diabetes. METHODS: Total of 192 patients were consecutively enrolled in the study who were admitted to coronary care unit because of acute coronary syndrome (ACS) and then underwent coronary angiography. MetS was defined by Iranian criteria. A coronary atherosclerosis score was used to quantify the extent of atherosclerotic involvement. The relationship between MetS and angiographic CAD severity or clinical presentation was compared between them after adjusting for diabetes. RESULTS: Individuals with MetS (n = 125) had a higher prevalence of ST-elevation myocardial infarction (71% vs 30%, P < 0.001), multi-vessel disease (50% vs. 34%, P = 0.003), decreased ejection fraction (P = 0.001) and more severe angiographic stenosis based on both modified Gensini (P = 0.081) and syntax (P = 0.008) scores, compared to those without MetS. Syntax score showed statistically significant difference between two groups before (P = 0.021) and after adjustment for diabetes (P = 0.005). CONCLUSION: MetS was related to the severity of CAD both clinically and by angiographic scores but diabetes was a challenging factor and may independently increase the severity of CAD.