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Background: The potential of iron to generate reactive oxygen species has motivated a long-standing interest in whether excess iron is causally linked to atherosclerotic heart disease. Circulating catalytic iron ("free" iron) is that which is not bound to transferrin or ferritin and is available to generate reactive oxygen species that may have deleterious vascular effects., Hypothesis: We hypothesized that increased levels of catalytic iron would be associated with increased cardiovascular events., Methods: We investigated the association of catalytic iron with clinical outcomes in 1701 patients with unstable angina, non-ST-segment elevation myocardial infarction (MI), or ST-segment elevation MI who were followed for a median of 10 months. All endpoints were adjudicated by a blinded Clinical End Points Committee., Results: The median catalytic iron level was significantly higher in those who died, 0.45 µmol/L (0.37, 0.57), compared with survivors, 0.37µmol/L (0.31, 0.46; P = 0.016). Catalytic iron was associated with a stepwise increased risk of death, with the highest quartile at an almost 4-fold risk compared with baseline (hazard ratio: 3.94, P = 0.035), which persisted after adjustment for age, diabetes, prior MI, prior congestive heart failure, ST-segment deviation, creatinine clearance, B-type natriuretic peptide, smoking, and Killip class (adjusted hazard ratio: 3.97, P = 0.036). There was no association between catalytic iron and risk of MI, recurrent ischemia, heart failure, or bleeding., Conclusions: Increasing catalytic iron levels were associated with increased all-cause mortality. Although our findings suggest that catalytic iron is not likely to add to available tools as a routine biomarker for risk stratification of recurrent ischemic events, its association with mortality is intriguing and leaves open the question of whether cardiovascular therapeutics aimed at catalytic iron may be useful. The TIMI Study Group has received research grant support from the Muljibhai Patel Society for Research in Nephro-Urology., (© 2013 Wiley Periodicals, Inc.)