BACKGROUND: The relevance of the adverse prognostic implications of CK-MB elevation after percutaneous coronary intervention (PCI) remains controversial. Therefore, we compared the relationship between the level of postprocedural CK-MB elevation and 6-month mortality in patients undergoing PCI with the relationship between the level of spontaneous, non-PCI-related CK-MB elevation and 6-month mortality in patients with acute coronary syndromes (ACS) treated medically. METHODS AND RESULTS: In the PURSUIT trial, 5583 of 9461 patients who presented with a non-ST-elevation ACS did not undergo PCI or CABG and had at least 1 CK-MB sample collected during index-hospitalization. There was a gradual increase in 6-month mortality with higher CK-MB levels: 4.1%, 8.6%, 9.0%, 14.3%, 15.5% for CK-MB ratios 0 to 1, >1 to 3, >3 to 5, >5 to 10, and >10 times the upper limit of normal. A combined analysis in 8838 patients undergoing PCI in 5 large, clinical trials revealed a proportional relationship between postprocedural CK-MB levels (1 to 3, >3 to 5, >5 to 10, and >10, the risk of death was 1.3%, 2.0%, 2.3%, 4.3%, and 7.4%, respectively. The absolute mortality rates were lower after procedure-related infarcts compared with spontaneous infarcts. Yet, the relative increase in 6-month mortality with each increase in peak CK-MB level was similar for PCI-related myocardial necrosis and spontaneous myocardial necrosis, as all tests for heterogeneity of the odds ratios were nonsignificant. CONCLUSIONS: The present analysis indicates that the adverse prognostic implications of periprocedural myocardial necrosis should be considered similar to the adverse consequences of spontaneous myocardial necrosis.