Myeloperoxidase (MPO) is associated with risk in acute coronary syndromes. However, the precise role it plays in ST-elevation myocardial infarction (STEMI) remains unclear. In this study we tested the hypothesis that levels of MPO in plasma after a myocardial infarction are affected by its ability to bind to the endothelium and there is local release of the enzyme at the culprit lesion. We measured plasma MPO in systemic circulation and throughout the coronary circulation in patients with STEMI undergoing primary percutaneous coronary intervention (PCI). MPO levels at the femoral artery were higher (p <0.001) in patients with STEMI (n = 67, median 45 ng/ml, interquartile range 34 to 83) compared to control patients (n = 12, 25 ng/ml, 19 to 30) with chronic stable angina undergoing elective PCI. After administration of the anticoagulant bivalirudin in 13 patients with STEMI, plasma MPO was increased only at the culprit coronary artery lesion before PCI (178 ng/ml, 91 to 245) versus all other sites (femoral artery 86 ng/ml, 54 to 139, p = 0.019). Administration of heparin caused a marked increase of plasma MPO. Even so, it was still possible to detect an increase of plasma MPO at culprit lesion in patients with STEMI (n = 54, 171 ng/ml, 122 to 230) versus controls (n = 12, 136 ng/ml, 109 to 151, p <0.05) after heparin and before PCI. MPO levels were higher at the culprit lesion in patients with STEMI who presented early and in those with restricted flow (p <0.05). In conclusion, our results demonstrate that, in addition to a systemic increase of MPO in patients presenting early with STEMI, levels of this leukocyte enzyme are increased at the culprit coronary lesion before PCI.