OBJECTIVE: To evaluate the levels of high mobility group box 1 protein (HMGB1) in serum of patients with hepatitis B virus (HBV)-related acute-on-chronic liver failure (ACLF) and investigate its potential relation to the clinical features of these patients. METHODS: Sixty patients with HBV-related ACLF, 30 patients with chronic hepatitis B (CHB), and 24 healthy individuals (controls) were enrolled in the study. Markers of liver function, such as aspartate aminotransferase (AST), were measured by routine biochemical methods. Imaging studies, such as abdominal computed tomography or magnetic resonance imaging, were used for disease staging. Serum levels of HMGB1 were measured by ELISA. Deaths within the 2-month follow-up after serum collection were used for the survival analysis. Patients who developed peritonitis, pneumonia, or other bacterial and fungal infections during the 2-month follow-up after serum collection were classified as the infected group. Pairwise comparisons were carried out by t-test, and multiple comparisons were carried out by analysis of variance. RESULTS: Patients with HBV-related ACLF had significantly higher serum levels of HMGB1 than CHB patients or controls (P = 0.003). Among the patients with HBV-related ACLF, those in the late stage (n = 20) had significantly higher levels of HMGB1 than those in the early stage (n = 20) (P = 0.005). The serum levels of HMGB1 correlated well with AST level in patients with HBV-related ACLF (P = 0.006). In addition, patients with HBV-related ACLF who developed infection or died during follow-up also had significantly higher levels of HMGB1 (P = 0.028 or P = 0.017, respectively). CONCLUSION: Enhanced serum level of HMGB1 is associated with development of HBV-related ACLF in CHB patients. The strong correlation between HMGB1 and AST levels suggest that HMGB1 may be useful as a prognostic marker for development of ACLF.