Objective: To investigate the serum level of serum high mobility group protein 1 (HMGB1) in patients with acute appendicitis and its diagnostic significance. Methods: The serum HMGB1 levels of 40 subjects undergoing health maintenance examination (group A) and 129 suspected acute appendicitis patients were detected by using quantitative ELISA kit, and their white blood cell (WBC) count and C-reactive protein (CRP) level were also determined. According to the surgical findings and postoperative pathological results, the 129 suspected cases were distinguished into group B (15 cases without appendicitis), group C (63 cases of simple acute appendicitis) and group D (51 cases of acute suppurative, gangrenous, perforated appendicitis or periappendiceal abscess). The differences in above mentioned indexes among the groups were compared and the receiver operating characteristic (ROC) curve analysis was performed to evaluate the diagnostic efficiency of each index for acute appendicitis. Results: The WBC count, serum level of HMGB1 and CRP markedly increased in group C and group D compared with group A or group B, and all differences had statistical significance (all P<0.05); further, all the values of the indexes in group D were significantly higher than those in group C (all P<0.05). ROC curve analysis showed that area under the curve (AUC) of WBC, CRP and HMGB1 were 0.729, 0.811 and 0.850 respectively, and HMGB1 had the highest diagnostic efficiency (both P<0.05). Conclusion: Patients with acute appendicitis have a pronounced elevation of serum HMGB1 level, thus it could be reasonably expected to become an important assistant diagnostic index for acute appendicitis and severity of this disease.