BACKGROUND: Plaque rupture of the atherosclerotic plaque is an important pathophysiologic mechanism of acute coronary syndrorne(acute myocardial infarction or unstable angina). Plaque rupture and resulting thrombus formation could be identified by intravascular ultrasound (IVUS), even though the sensitivity was variable in previous reports. We sought to know the morphologic characteristics, incidence of plaque rupture and thrombus formation by ultrasound in patients with acute coronary syndrome. METHOD: Between April and Septernber 1997, 23 admitted patients who was diagnosed as unstable angina or acute rnyocardial infarction was included. We performed coronary angiography with IVUS examination within 2 weeks. Atherosclerotic plaque was classified into soft, fibrous, calcific, and mixed plaque, and plaque rupture was defined as rupture of fibrous cap with discontinuity and / or backflow into plaque. Thrombus was defined as a scintillating, movable mass or layering materials that could be distinguished from underlying plaque. RESULTS: Most of the plaques were soft and mixed types(14 and 6, out of 23 cases). Plaque rupture could be identified only in small portion(6 cases, 26%) of the cases. Thrombus was noted in 12 cases(52%). 4 cases showed both plaque rupture and thrombus. CONCLUSION: Soft plaque is the most frequent plaque pye in acute coronary syndrome. IVUS is a useful tool to identify the morphologic features of the plaque such as rupture and thrombus formation.