Objective To observe the protective effect of acetylcysteine against radiation pneumonia.Methods Total of 80 patients who were inoperable were randomly allocated into treatment group and control group.Using conformal radiation technology and the total dose was 65 ~ 75Gy.The patients in treatment group were given acetylcysteine and radiotherapy;the patients in control group were given radiotherapy only.Results All patients were treated radiotherapy.The effective rate( CR and PR) of treatment group was 90%,and that of control group was 85%(P > 0.05);The incidence of acute radiation pneumonia and pulmonary fibrosis in treatment group were 15% and 20%,respectively;and that of control group were 33% and 45% respectively.There was significant difference between the two groups ( P < 0.05 ).Conclusion Using acetylcysteine during radiotherapy could prevent radiation pneumonia in the non-small cell lung cancer patients. Key words: Acetylcysteine; Carcinoma, non-small-cell lung; Radiotherapy, conformal; Radiation pneumonia/pulmonary fibrosis