BACKGROUND: Inflammation has an important role in the development and progression of atherosclerosis, and acute phase proteins such as pentraxin-3 (PTX3) can be deployed in determining the prognosis of coronary artery disease (CAD). So the purpose of this paper was to evaluate the PTX3 level and its related factors in patients undergoing primary percutaneous coronary intervention (PCI). METHODS: In this cross-sectional study, the PTX3 levels were determined for 100 patients with ST-elevation myocardial infarction referred to the Modarres Hospital, Tehran, Iran. Checklist included demographic data [age, gender, history of myocardial infarction (MI)] and characteristics of heart disease (type of MI, culprit, and pre-dilation). PTX3 was measured for all patients before PCI. RESULTS: In this study, the mean age of the participants was 58.7 (11.4). Global registry of acute coronary events (GRACE) score was higher in the group with abnormal PTX3 levels (P = 0.008). The number of the involved vessels (P = 0.005), MI type (P = 0.05), and the need for PCI all had a significant relation with abnormal PTX3 levels. The increased levels of PTX3 received higher Killip class, lower ejection fraction, and higher GRACE score. The group with abnormal PTX3 had a significant difference in platelet counts (P = 0.018) in comparison with the group with normal level of PTX3. CONCLUSION: Currently, the biomarkers are highly important in the field of cardiovascular diseases. The diagnostic and prognostic importance of PTX3 as a new marker has been underscored in recent studies. Differentiating between high-risk patients with acute cardiac infarction and low-risk ones through their clinical signs is difficult.