The aim of this study is to explore the impact of metabolic syndrome (MS) on the outcome of patients with non-clinically diagnosed diabetes with acute coronary syndrome (ACS) based on a comprehensive nationwide registry during a 1-year follow-up. In the ACS Israeli Survey, 1,060 consecutive patients with non-clinically diagnosed diabetes were admitted due to ACS; 359 patients with MS features on admission were compared with 701 subjects without MS. A modified National Cholesterol Education Program Adult Treatment Panel III definition of MS was used in patients who presented with ≥3 of the 5 components: (1) hyperglycemia, defined as occasional blood glucose on admission >140 mg/dl; (2) preexisting hypertension; (3) body mass index >28 kg/m2; (4) high-density lipoprotein cholesterol ≤40 mg/dl (men) or ≤50 mg/dl (women); and (5) triglycerides ≥150 mg/dl. Patients with MS were more frequently women (27% vs 12%, p = 0.001), were in Killip ≥II on admission (19% vs 14%, p = 0.03), and had higher 30-day (5.0% vs 1.7%, p = 0.002) and 1-year (8.9% vs 4.6%, p = 0.005) crude mortality rates. Patients with hyperglycemia (glucose >140 mg/dl) and MS had higher 30-day mortality rates compared with patients with hyperglycemia without MS (8.3% vs 2.5%, p <0.05). Multivariate analysis identified MS as a strong independent predictor of 30-day and 1-year mortality with hazard ratios of 2.54 (95% confidence interval 1.22 to 5.31) and 1.96 (95% confidence interval 1.18 to 3.24), respectively. In conclusion, MS defined early at admission is a strong independent predictor of mortality and morbidity in patients with non-clinically diagnosed diabetes with ACS.