['Some patients with acute coronary syndrome (ACS) have large myocardial infarction but others have small or no infarction. However, what makes this difference has not been clarified. We compared the angioscopic findings between those two categories of ACS patients and examined the association between the severity of ACS and the morphology of both culprit lesion and nonculprit coronary segments.', 'Prospectively and consecutively enrolled patients with ACS were classified as CK-elevation-ACS (CKE-ACS; n = 54) or non-CK-elevation-ACS (NCKE-ACS; n = 22). Patients were diagnosed as CKE-ACS when the elevation (greater than twice the normal upper limit) of CK-MB was detected; otherwise, patients were diagnosed as NCKE-ACS. They all underwent emergent catheterization and PCI of the culprit lesion. The entire culprit artery was observed by angioscopy, and the prevalence of thrombus and the color grade of yellow plaques were evaluated. The color grade of yellow plaques were classified as 0 (white), 1 (slight yellow), 2 (yellow), or 3 (intense yellow) according to the standard colors. The color grade of culprit plaque (CC), number (NP) and maximum (MC) color grade of yellow plaques in the nonculprit segments, plaque index (PI = N x MC), and prevalence of thrombus at the culprit lesion (CT) and in the nonculprit segments (NT) were compared between CKE-ACS and NCKE-ACS patients.', 'CC (1.9 +/- 0.9 vs. 1.7 +/- 0.8; p = 0.3) and CT (93% vs. 77%; p = 0.06) were not significantly different between CKE-ACS and NCKE-ACS patients, however, NP (2.2 +/- 1.6 vs. 1.4 +/- 1.2; p = 0.03), MC (1.8 +/- 0.9 vs. 1.2 +/- 0.9; p = 0.008), PI (4.8 +/- 4.4 vs. 2.4 +/- 3.1; p = 0.03), and NT (39% vs. 11%; p = 0.02) were significantly higher in CKE-ACS than in NCKE-ACS patients.'] Is angioscopically-determined extent of coronary atherosclerosis associated with severity of acute coronary syndrome?