Aim To investigate the histopathological characteristics of thrombus aspirated during primary percutaneous coronary intervention （PCI） in a consecutive ST-segment elevation myocardial infarction （STEMI） population.Methods Thrombus aspiration during primary PCI was performed in 329 STEMI patients. Thrombus age was classified as fresh （＜1 day），lytic （1-5 days），or organized （＞5 days）. The histopathological findings were related to the clinical，angiographic，and procedural characteristics. Results Thrombus could be aspirated in 234 patients （71%）. Fresh thrombus was found in 133 of 230 patients （58%） compared to 97 patients （42%） with lytic or organized thrombi.Conclusion In 42% of patients thrombus age is older than 24 hours，indicating that plaque disruption and thrombus formation occur significantly earlier than the onset of symptoms in many patients.