Objective: To evaluate the role of MHC Ⅱ expression in posttrauma immune disturbances and infections.Methods: In experimental study, peritoneal macrophages were harvested from traumatized mice and treated with GM-CSF, Ia molecules were determined by flow cytometry. In 24 trauma patients, monocyte HLA-DR expression was measured by APAAP technique.Results: Marked inhibition of MHC Ⅱ molecule expression was found in both traumatized mice and patients. In traumatic patients with infection, the inhibition was more severe. Treatment with GM-CSF in mice partially restored the Ia expression.Conclusions: Inhibition of MHC Ⅱ molecules plays an important role in postrauma immune disturbances.There is a close relationship between HLA-DR inhibition and clinical infections.