Atherosclerotic plaque rupture is the main cause of acute coronary syndromes. There is no systematic description of ruptured plaques located in the left main coronary artery (LMCA). The aim of our study was to describe ruptured plaques in the LMCA with intravascular ultrasound. We identified 17 LMCA plaque ruptures in 16 patients. Two patients had a recent myocardial infarction, 13 had unstable angina, and 1 had stable angina. Two left main ruptures were clinically considered to be incidental findings and were left untreated; the remaining 14 were culprit lesions that were treated with emergency coronary artery bypass grafting (n = 3) or LMCA stenting (n = 11). One patient died in the hospital after left main stenting for unstable angina; all other patients who underwent stenting or surgical bypass did well. Almost all ruptured plaques that were identified by intravascular ultrasound had an angiographic complex appearance that was defined by the presence of ulceration (81%), intimal flap (38%), aneurysm (12%), or thrombus (6%). All ruptures were located in the distal half of the LMCA; 6 ruptures involved bifurcation and 11 were confined to the LMCA proper. The angle between the maximum plaque cavity and the left anterior descending/left circumflex artery flow divider was 162.3 degrees +/- 15.6 degrees in bifurcation versus 71.3 degrees +/- 41.6 degrees in nonbifurcation lesions (p <0.0001). The LMCA minimum lumen area measured 6.0 +/- 3.3 mm2 in nonbifurcation lesions and 7.4 +/- 4.7 mm2 in bifurcation lesions (p = 0.5). LMCA plaque ruptures mostly present as unstable angina, are located in the distal portion and/or bifurcation of the LMCA, often do not compromise the lumen, and have an angiographic complex appearance. When ruptured plaques involve the bifurcation, they occur opposite the distal flow divider.