Abstract: Objective To study the value of high mobility group box chromosomal protein 1（HMGB1） for diagnosis of polymyositis/dermatomyositis（PM/DM） by measuring its serum level in patients with PM/DM. Methods Forty-seven PM/DM patients,41 patients with SLE,37 patients with other rheumatic diseases and 50 healthy volunteers were enrolled in this study.Their HMGB1 level was measured by ELISA and correlation between HMGB1 level and other rheumatic diseases was analyzed. Results The serum HMGB1 level was significantly higher in patients with PM/DM and SLE than in healthy volunteers（22.65pg/ml vs 16.76pg/ml and 7.24pg/ml, P＜0.01）,and in patients with PM/DM than in those with SLE and other rheumatic diseases（22.65ng/ml vs 16.76ng/ml and 12.39ng/ml, P＜0.05）.Logistic regression analysis showed that the serum HMGB1 level was correlated with PM/DM. Conclusion The serum HMGB1 level is significantly higher in patients with PM/DM than in those with other rheumatic diseases,indicating that HMGB1 may play an important role in the pathogenesis of PM/DM and can therefore be used as a diagnostic marker of PM/DM.