We assessed culprit lesions of infarct-related arteries (IRA) using Virtual Histology Intravascular Ultrasound (VH-IVUS) in 112 pts with acute ST elevation myocardial infarction (STEMI). Methods. STEMI pts were divided into 2 groups: IRA with total occlusion (TOI, n=67) and partial-occluded IRA(POI, n=45). Thrombolysis in myocardial infarction (TIMI) thrombus grade was evaluated. VH-thin-capped fibroatheroma (VH-TCFAs) was defined as necrotic core (NC) >10% of plaque area, plaque burden >40%, and NC in contact with the lumen for ���3 image slices. Positive remodeling was defined as lesion/reference external elastic membrane >1.05. Results. All TOI showed TIMI thrombus grade 5; however, grade 1 thrombus was most frequent in POI (grade 0, n=5; grade 1, n=17; grade 2, n=12; grade 3, n=9; grade 4, n=2; and grade 5, n=0). Vessel size, lesion length, plaque burden, minimal lumen area (MLA), and remodeling index were similar in TOI vs POI (Table). Plaque rupture was present in 48 pts: 27 TOI and 21 POI (p=0.3). Ruptured plaque cavity size was similar (0.81짹0.78mm2 in TOI vs 0.93짹0.81mm2 in POI, p=0.7). The incidence of VH-TCFAs tended to be higher in TOIs [51% (34/67)] vs POI [27%(12/45), p=0.12]; %dense calcium within the culprit lesions was larger in TOI vs POI (p=0.04); and peak troponin I was higher in TOI than POI (366짹218 mg/dl vs 156짹260 mg/dl, p=0.012). Conclusions. Totally-occluded IRAs in STEMI pts are associated with more culprit lesion calcium and VH-TCFA morphology and greater myonecrosis indicating a higher risk profile.