Background and aims It is well known that the interaction between platelets (PLTs), endothelial cells, and leukocytes contributes to thrombosis in patients with acute coronary syndrome. The aim of this study was to investigate the significance of PLTs and eosinophils (EOS) in coronary arterial thrombi. Methods PLT count, mean PLT volume, PLT mass, EOS count, EOS percentage, and troponin I level in peripheral blood were determined in 81 patients with angina pectoris (AP) and 49 patients with acute myocardial infarction (AMI). A total of 12 thrombus specimens from AMI patients were submitted for histopathological analysis. EOS presence in thrombectomy specimens were checked by hematoxylin-eosin staining and confirmed by Luna staining. Results Results showed that EOS were present in all 12 samples (100%). Cell count and percentage of EOS in peripheral blood of patients with AMI were lower than those in patients with AP (both P<0.00001). A higher PLT count was observed in AMI patients (243 +/- 70), especially among female patients or those who were older than 60 years, when compared with AP patients (216 +/- 60; all P<0.05). According to the troponin I level, we divided AMI patients into groups I (>= 20 ng/ml) and II (<20 ng/ml). Group I had a lower EOS percentage compared with group II (P=0.0496). PLT count was also lower in group I with no statistical difference found (P=0.1202). Moreover, there was an inverse correlation between the EOS percentage and the troponin I level (r=-0.434). Conclusion In conclusion, patients with AMI presented with a decreased EOS percentage and an increased PLT count. The decreased EOS percentage suggested serious myocardial damage. The study indicated that EOS play an important role in thrombosis in patients with acute coronary syndrome. Coron Artery Dis 26: 101-106 Copyright (C) 2015 Wolters Kluwer Health, Inc. All rights reserved.