Background ST-elevated acute myocardial infarction (STEAMI) happening in the first month post percutaneous coronary intervention (PCI) is almost related to acute thrombosis or subacute thrombosis in-stents. This study aimed to investigate the possible causes of myocardial infarction one month later. Methods Patients who had a history of successful PCI, and received coronary angiography or re-PCI due to STEAMI were included in this study. The AMI-related lesions and previous angiographic findings such as the number of lesions, the degree of the stenosis, the type of stents and acute results of last PCI were recorded. If the AMI-related lesion was localized in-stents or at the edge of stents (distance apart from the edge <5 mm), it was defined to be late thrombosis; otherwise as a new-lesion induced AMI. Results One hundred and ninety-two patients aged 40-79 years were included in this study. New lesions, as the cause of STEAMI, were found in 144 patients (Group A, 75%), and late thrombosis in 48 patients (Group B, 25%). Almost all newly built thromboses were found at the sites of previous insignificant lesions (diameter stenosis <50%). There was a significant difference in the average time from previous PCI to AMI ((30.1+12.4) vs (20.3+11.9) months) between the two groups. Diabetes mellitus (DM) and drug-eluting stent (DES) utilization were associated with markedly higher morbidity of late thrombosis in adjusted Logistic regression (hazard ratio (HR) 3.4, 95% confidence interval (CI) 1.1-10.9 and 5.3, 95% CI 1.1-26.5). Conclusions STEAMIs happening 1 month after PCI are more likely to develop from previous insignificant lesion rupture than from late thrombosis in-stents. Moreover, DM and DES are associated with the high incidence of latethrombosis, which may indicate that intensive antiplatelet therapy should be considered in patients with diabetes.