Background/Purpose: Coronary vasospasm causes not only transient angina (variant or Prinzmetal���s angina) but also acute myocardial infarction, ventricular tachycardia and sudden cardiac death. However until now, its precise mechanism of occurrence has not been clarified. And it is also unknown which intracoronary pathologic lesions may contribute to it. The aim of study is to evaluate morphologic changes of vasospastic lesions which may be culprits causing acute coronary syndromes using a novel technique, optical coherence tomography (OCT). Methods: Eight patients (40~83 years old, all males) with vasospasm induced acute coronary syndrome who visited emergency room because of continuous chest pain and transient ST segment elevation in electrocardiogram were enrolled for the study. All patients showed no significant coronary artery disease and the spastic lesions were dilated by intracoronary nitroglycerin injections. Optical coherence tomography examinations were performed for evaluation of vasospastic culprit lesions. Results: Intima tears, intima erosions and intraluminal thrombi were found in 5 patients (62.5%), 7 patients (87.5%) and 3 patients (37.5%) respectively. Serum cardiac troponin-I level was 3.03 짹 5.69 ng/ml for patients with intima tear versus 0.02 짹 0.01 ng/ml for patients without intima tear. (p< 0.05) And high senstivity-C reactive protein level was 3.38 짹 3.29 mg/L for patients with intima tear versus 0.36 짹 0.11 mg/L for patients without intima tear. (p< 0.05) The creatinine kinase – myocardial band isoenzyme and total cholesterol level were not significantly diffenrent. The other parameters including pain duration, residual stenosis, lesion length and coronary risk factors were not significantly different between groups with and without intima tear. Conclusion: In patients with vasospasm induced acute coronary syndrome, intima tears, intima erosions and intraluminal thromboses were major morphologic findings of optical coherence tomography examinations. Key words: Acute coronary syndrome, optical coherence tomography (OCT), intima tear, intima erosion, vasospastic angina