Introduction. Hepatic ischemia followed by reperfusion (I/R) occurs in the settings of trauma, transplantation, and elective liver resections. The mechanisms that account for local organ damage are only partially understood. High-mobility group box 1 (HMGB1) has been identified as a late mediator of lethality in sepsis and is released by damaged tissues where it could also act as a mediator of inflammation and organ injury. We hypothesized that HMGB1 would contribute to organ injury following hepatic I/R. Methods. HMGB1 protein expression and hepatocellular secretion were determined in rat hepatocytes subjected to hypoxia (1% O 2 ) versus normoxic cells. The role of HMGB1 in mediating hepatic I/R injury was examined in C57Bl/6 mice that underwent 90 min of partial I/R injury. These mice were pretreated with neutralizing HMGB1 antibody (600 μg) or control normal saline 1 h prior to ischemia. Results. Basal HMGB1 expression was observed in normoxic hepatocytes and was dramatically up regulated 12-24 h after hypoxia. This corresponded to increased HMGB1 secretion 18-24 h after hypoxia. In mice undergoing partial warm liver I/R injury, HMGB1 protein expression is increased as early as 1 h after reperfusion and then increases in a time-dependent manner up to 24 h. Inhibition of HMGB1 activity with neutralizing antibody significantly decreased liver damage after warm ischemia compared to control animals. TABLE-ABSTRACT 81GroupALT-1 hALT-6 hALT-24hSham67 +/- 2457 +/- 851 +/- 2Control3101 +/- 11678140 +/- 1656905 +/- 65Anti-HMGB1662 +/- 1202382 +/- 687265 +/- 16Note. Data are mean +/- SEM, n = 3-4 per group;*indicates P < 0.05 versus control. Conclusion. This study demonstrates that HMGB1 is up regulated in hepatocytes by simple hypoxia. The same was seen to occur in vivo in liver I/R as early as 1 h. The reduction in liver damage by a neutralizing antibody shows that HMGB1 is a key mediator in the pathways that lead to organ damage in warm liver I/R. Thus, unlike sepsis where HMGB1 is late mediator, this danger protein appears to be an early mediator in I/R.