Background: Optical coherence tomography (OCT) is a high-resolution imaging method which can identify plaque rupture and a thin-cap fibroatheroma (TCFA) more accurately than grey-scale intravascular ultrasound. The aim of this study is to estimate the axial distribution of plaque rupture and TCFA within the coronary atherosclerotic culprit lesion using OCT in patients with acute coronary syndrome (ACS). Methods: We evaluated thrombus, plaque rupture and TCFA in 27 ACS patients (6, acute myocardial infarction: 21, unstable angina) who underwent pre-intervention OCT study. The minimal lumen area (MLA) site and the location of plaque rupture and TCFA were analyzed. TCFA was defined as a plaque with lipid content in ��� 2 quadrant and the thinnest part of the fibrous cap measuring < 65���.Results: Twenty-one lesions are left anterior descending artery. The average pullback length is 33.7 짹 8.2 mm and average lesion length is 17.4 짹 6.3 mm. Thrombi are observed in 19 patients (70.4%), total 19 (70.4%) plaque ruptures are detected and multiple plaque ruptures are detected in two patients(7.4 %). TCFA is detected in 14 (51.9%) cases. Plaque rupture is located proximal to MLA site in 68.4% (by average 2.9 mm) and distal in 31.6% (by average 2.5 mm) of lesions (p= 0.108). TCFA is more common proximal to MLA site in 78.6% (by average 2.9 mm) and distal in 21.4% (by average 1.6. mm) of the lesions (p= 0.033). Conclusion: In OCT study, plaque ruptures and TCFAs are frequently observed within the culprit lesion in ACS patients and TCFAs are more common proximal to MLA stie.