Rationale: Mechanisms of coronary occlusion in ST-elevation acute coronary syndrome are poorly understood. We have previously reported that neutrophil (polymorphonuclear cells [PMNs]) accumulation in culprit lesion site (CLS) thrombus is a predictor of cardiovascular outcomes. Objective: The goal of this study was to characterize PMN activation at the CLS. We examined the relationships between CLS neutrophil extracellular traps (NETs), bacterial components as triggers of NETosis, activity of endogenous deoxyribonuclease, ST-segment resolution, and infarct size. Methods and Results: We analyzed coronary thrombectomies from 111 patients with ST-elevation acute coronary syndrome undergoing primary percutaneous coronary intervention. Thrombi were characterized by immunostaining, flow cytometry, bacterial profiling, and immunometric and enzymatic assays. Compared with femoral PMNs, CLS PMNs were highly activated and formed aggregates with platelets. Nucleosomes, double-stranded DNA, neutrophil elastase, myeloperoxidase, and myeloid-related protein 8/14 were increased in CLS plasma, and NETs contributed to the scaffolds of particulate coronary thrombi. Copy numbers of Streptococcus species correlated positively with dsDNA. Thrombus NET burden correlated positively with infarct size and negatively with ST-segment resolution, whereas CLS deoxyribonuclease activity correlated negatively with infarct size and positively with ST-segment resolution. Recombinant deoxyribonuclease accelerated the lysis of coronary thrombi ex vivo. Conclusions: PMNs are highly activated in ST-elevation acute coronary syndrome and undergo NETosis at the CLS. Coronary NET burden and deoxyribonuclease activity are predictors of ST-segment resolution and myocardial infarct size.