Cerebral and myocardial ischemia, two of the leading causes of morbidity and mortality worldwide, are associated with inflammation that can lead to multiple organ failure and death. High-mobility group box 1(HMGB1), a recently described mediator of lethal systemic inflammation, has been detected in individuals with severe sepsis and hemorrhagic shock, but its role during ischemic injury in humans is unknown. To determine whether systemic HMGB1 levels are elevated after ischemic injury, a prospective observational study was performed in subjects with a diagnosis of either Acute Coronary Syndrome (ACS) or cerebral vascular ischemia (transient ischemic attack or cerebral vascular accident). Subjects (n, 16; age [mean], 67+/-16.3 years) were enrolled in the North Shore-LIJ emergency department within 24 h of symptom onset. Blood samples were collected, and HMGB1 levels analyzed by Western blot analysis using previously described methods (Wang et al. Science. 1999). Control samples were obtained from healthy age- and sex-matched volunteers (n, 16; age [mean], 68+/-15.8 years). Here, we report that serum HMGB1 levels were significantly elevated in both myocardial ischemia subjects (myocardial control serum HMGB1, 1.94+/-2.05 ng/mL, vs. myocardial ischemia serum HMGB1, 159+/-54.3 ng/mL; P<0.001); and in cerebral ischemia subjects (cerebral control serum HMGB1, 16.8+/-10.9 ng/mL, vs. cerebral ischemia serum HMGB1, 218+/-18.8 ng/mL; P<0.001). These results suggest that systemic HMGB1 levels are elevated in human ischemic disease.