Acute lung injury (ALI) is characterized by alveolar injury and uncontrolled inflammation. Mechanisms underlying pathogenesis of ALI are unknown. Regulatory T cells (Tregs), either natural or induced, suppress a variety of physiological and pathological immune responses. In the current study, we investigated whether Tregs were involved in the development of ALI. Proportion of CD4+CD25+FoxP3+ Tregs in the peripheral blood of 66 ALI patients and 30 healthy controls were examined by flow cytometry. Data showed that the percentage of Tregs in CD4+ T cells was significantly increased in patients than that in controls (10.8 versus 7.6%, P=0.003). Also, compared to those who died during the study, patients who survived presented significantly higher level of Tregs at the time of recruitment (P=0.041). Since Tim-3 is a negative regulatory molecule and can modulate the function of Tregs, we evaluated Tim-3 level on Tregs and identified upregulation of the molecule in patients than that in controls. Moreover, compared to those who died during the study, patients who survived showed 1.7-fold higher level of Tim-3 on Tregs at the time of recruitment (P<0.001). These results suggest that Tregs could affect the prognosis of ALI probably due to the upregulation of Tim-3.