Background: Thrombus propagation on disrupted atherosclerotic plaque leads to acute myocardial infarction (AMI). Recent studies have shown that the histopathology of the coronary thrombus is associated with myocardial reperfusion, ST-segment recovery, distal embolization and long-term mortality in patients with AMI. Therefore, we investigated the histopathologic characteristics of material aspirated during percutaneous coronary intervention (PCI) in patients with AMI, and assessed whether the histologic findings are related to in-hospital mortality. Methods and Results: In this prospective single-center registry, coronary materials were obtained during PCI from 264 AMI patients within 24h of the onset of anginal symptoms. Organized thrombus, calcification and plaque components in the aspirated material were morphologically assessed. In-hospital deaths occurred in 17 (6%) patients. Organized thrombi were found in 91 (34%) of 264 patients, calcification was identified in 44 (17%) and plaque components in 117 (44%) patients. Rates of in-hospital all-cause mortality were significantly higher among patients with than without organized thrombus (P<0.05). Multivariate analysis also identified organized thrombus as an independent predictor of in-hospital death, as well as age, a history of myocardial infarction and the presence of shock (P<0.05). In contrast, calcification and plaque components were not significantly associated with in-hospital mortality. Conclusions: Our results suggest that organized thrombus in aspirated coronary material is an independent predictor of in-hospital mortality of patients with AMI.  (Circ J 2013; 77: 1275–1280)