Objective: The aim of this study is to identify serum copeptin levels in patients diagnosed with unstable angina (UA) and to evaluate the relationship between copeptin levels and angiographic severity of the patients. Materials and Methods: Two hundred patients who had been diagnosed with UA and undergone coronary angiography were included in the study. Each patient underwent a clinical evaluation, including a 12-lead electrocardiogram, echocardiographic evaluation, laboratory tests (high sensitive troponin-T and copeptin level tests) and The Global Registry of Acute Coronary Events (GRACE) 1.0 risk score calculation at the time of admission. Synergy Between Percutaneous Coronary Intervention With Taxus and Cardiac Surgery (SYNTAX-1) score was calculated after coronary angiography. Results: We isolated and defined two subgroups within our study population: group 1 included patients with non-significant coronary artery disease (CAD) (<50% diameter stenosis, n:105); group 2 included patients with significant CAD (≧50% diameter stenosis, n:95). The number of cases with a GRACE score higher than 140 was significantly higher in group 2 than in group 1 (p<0.001). SYNTAX scores and copeptin levels were significantly higher in group 2 than in group 1 (p<0.001 for both). A positive correlation was detected between copeptin levels and SYNTAX scores (r = 0.683; P < 0.001), and the cutoff level of copeptin level was 18.3 pmol/l (sensitivity of 74.7 %, specificity of 83.8% and area under curve of 0.795). Conclusion: Our study suggests that it may be beneficial to use both conventional scoring systems and serum copeptin levels when attempting to identify high-risk UA patients.