Objective To study the effect of low-dose cyclophosphamide (CY) on apoptosis of lung parenchyma cells in the early severe burn stage in rats. Methods Ninety clean SD male rats were randomly divided into 3 groups: the normal group (n=10), the experimental group (n=40) and the burn group (n=40). The model of degree III with 30% burn area was made in the experimental group and the burn group. CY (2 mg/kg) was injected into the abdominal cavity right after burn in the experimental group. No treatment was done in the normal group and burn group. Lung tissues were obtained at 3, 6, 12and 24 hours, respectively, after burn, and were observed by HE staining. Apoptosis of lung parenchyma cells was observed by TUNEL. Results Lung tissues were observed under the opticalmicroscopy in the normal group: the pulmonary structure was clear, and there were no inflammatory cells and exudation in the alveolar space and bronchial lumen. Besides, a few RBCs were seen. Pathological changes of lung tissues were observed under the opticalmicroscopy in the burn group: alveolar septum was obviously widened; alveolar wall was destroyed; interstitial edema and atelectasis occurred; and pathological lesion was gradually aggravated as time passed by. The pathological lesion of lung tissues mentioned above in the experimental group was better than those in the burn group. Compared with the normal group, the apoptosis ratio of lung parenchyma cells continuously increased in the burn group from the 3 hour after burn, and reached the peak at 12 hours. There were significant differences between the two groups (P lt; 0.05). However, in the experimental group, the apoptosis ratio of lung parenchyma cells increased at 3 hours after burn, cut down to normal at 6 and 12 hours, respectively, and notably decreased at 24 hours. There were significant differences between the experimental group and the normal group (P lt; 0.05). Compared with the burn group, the apoptosisrate of lung parenchyma cells in the experimental group began to decrease strikingly from the 6 hours after burn, and there were significant differences between the two groups (P lt; 0.05). Conclusion Low-dose CY can restrain the apoptosis of lung parenchyma cells in the early severe burn stage in rats and alleviate the injury of the lung.