Plasma levels of high mobility group box chromosomal protein-1 (HMGB-1), as well as of other inflammatory molecules such as interleukin-6 (IL-6), regulated on activation normal T-cell expressed and secreted (RANTES), and soluble intercellular adhesion molecule-1 (sICAM-1), were determined in patients with bacterial pneumonia coinfected with influenza virus. HMGB-1 levels were significantly elevated in these patients compared to patients undergoing mild bacterial pneumonia alone (p < 0.01). Among cases of coinfection, we found a significant correlation between the concentration of HMGB-1 and white blood cell counts (p < 0.05, r = 0.612). Levels of IL-6 were also higher in these patients than in patients with bacterial pneumonia alone (p < 0.05), despite similar levels of RANTES and sICAM-1 in the 2 groups. These data suggest that HMGB-1 is involved in the pathogenesis of severe bacterial pneumonia coinfected with influenza virus.