OBJECTIVES: The aim of this study was to evaluate the relative impact of spontaneously occurring and periprocedural myocardial infarction (MI) on survival after percutaneous coronary intervention (PCI). BACKGROUND: The clinical significance of periprocedural MI after PCI remains uncertain. METHODS: Outcomes during a 1-year follow-up were evaluated among 7,773 patients enrolled in the ACUITY (Acute Catheterization and Urgent Intervention Triage Strategy) trial with a non-ST-segment elevation acute coronary syndrome in whom PCI was performed. RESULTS: Periprocedural MI developed in 466 patients (6.0%), and spontaneous MI unrelated to PCI subsequently developed in 200 patients (2.6%). Patients developing spontaneous and periprocedural MI compared with those patients without MI had significantly greater unadjusted rates of mortality at 30 days (5.0% vs. 3.2% vs. 0.8%, respectively, p < 0.0001) and at 1 year (16.0% vs. 6.0% vs. 2.6%, respectively, p < 0.0001). In a time-updated multivariable analysis, after adjusting for differences in baseline and procedural characteristics between the groups, we found that spontaneous MI was a powerful independent predictor of subsequent mortality (hazard ratio: 7.49, 95% confidence interval: 4.95 to 11.33, p < 0.0001), whereas periprocedural MI was not a significant predictor of mortality (hazard ratio: 1.30, 95% confidence interval: 0.85 to 1.98, p = 0.22). CONCLUSIONS: Among patients with acute coronary syndrome undergoing PCI, the spontaneous development of an MI unrelated to PCI is a powerful predictor of subsequent mortality. In contrast, periprocedural MI is a marker of baseline risk, atherosclerosis burden, and procedural complexity but in most cases does not have independent prognostic significance. (Comparison of Angiomax Versus Heparin in Acute Coronary Syndromes [ACS]; NCT00093158).