Bronchiolitis has been known as one of the pathological features of lung injuries in mustard gas exposed patients.The purpose of this research was to evaluate the efficacy of interferon gamma-1b on the lung function in mustard gas exposed patients with bronchiolitis. In multicenter research interferon gamma-1b was effective in pulmonary fibrosis with unknown reason, but assessment of effect of interferon gamma-1b in the chemical injured patients has not so far been reported.Thirty six patients with bronchiolitis whose lung lesion had been diagnosed through the chest high resolution computerized tomography (HRCT) and pathological study were divided into two eighteen member case and control groups. The case group was treated for 6 months with a combination of 200 mug of interferon gamma-1b (given three times per week subcutaneously) and 7.5mg of prednisolone (given once a day), while the control group received their previous medications (prednisolone 7.5mg/day + salbutamol and beclomethasone spray PRN).In the two groups, FEV1 did not have statistical differences at base line (49.3+/-2.9 and 48.7+/-4.1, respectively, p = 0.6), whereas after treatment the data for FEV1 showed a significant increase in the case group (66.3+/-5.4) when compared with those in the control group (57.3+/-8.6, p=0.001). The findings of this study indicate that a 6-month treatment with interferon gamma -1b plus a low-dose of prednisolone is associated with the improvement of the lung function in mustard gas exposed patients with bronchiolitis.