Aims: Clinical evidence indicates that innate immune cells may contribute to the onset and outcome of acute coronary syndrome (ACS). Our prospective study aimed at analysing neutrophil phenotypes in ACS and their role in predicting 1-year major cardiovascular events. Methods: Blood neutrophil phenotypes were analysed by flow cytometry. Differential blood cell count and plasma levels of soluble markers were recorded at admission and at 6-month follow-up. Results: 108 patients categorized in chronic stable coronary artery disease (n=37), unstable angina (UA) (n=19), Non-ST-Elevation Myocardial Infarction (NSTEMI) (n=25), and ST-Elevation Myocardial Infarction (STEMI) (n=27) were included. STEMI and NSTEMI patients displayed higher neutrophil count and neutrophil-to-lymphocyte ratio (NLR) than stable and UA patients (P<0.0001), which normalized at 6-month after MI. STEMI patients were characterized by elevated percentages of band cells in low-density neutrophils (P=0.007) and in high-density neutrophils (P=0.019) compared to the other patients. Multivariable logistic regression analysis revealed that plasma levels of total MPO was associated with STEMI when compared to stable (OR: 1.434; 95% CI: 1.119-1.837; P<0.0001), UA (1.47; 1.146-1.886; P=0.002), and NSTEMI (1.213; 1.1-1.134; P=0.0001) patients, while increased neutrophil SSC signal intensity was associated with NSTEMI compared to stable patients (3.828; 1.033-14.184; P=0.045). Based on multivariable Cox regression analysis, elevated plasma levels of PCSK9 and low-density neutrophil percentage predicted 1-year outcome independently of cardiovascular risk factors (c-index: 0.915; IQR: 0.908-0.929). Conclusions: Changes in neutrophil phenotype are concomitant to ACS. These changes may differ between STEMI and NSTEMI. They may also contribute to ACS risk and patient outcome.