Administration of N-acetylcysteine may be effective in diseases caused by oxidative-antioxidative imbalance. We aimed to determine the effect administration for 4 months of N-acetylcysteine (1200 mg daily) on sulfur mustard-induced bronchiolitis obliterans in patients with normal pulmonary function test. In a double-blind clinical trial, 144 patients with bronchiolitis obliterans due to sulfur mustard and bronchiolitis obliterans syndrome class 0, randomly entered to group 1 (n = 72, N-acetylcysteine) and group 2 (n = 72, placebo). The changes in dyspnoea, wake-up dyspnoea, cough and sputum were measured after 4 months using a 'delta value' (i.e. symptom score after 4 months--symptom score before the trial). Spirometric findings were measured at the beginning of the trial, 2 months later and 4 months later. Dyspnoea (delta value: -0.78 (0.61), P < 0.001), wake-up dyspnoea (delta value: -0.57 (0.64), P < 0.001), and cough (delta value: -0.86 (0.63), P < 0.001) improved after 4 months of N-acetylcysteine administration compared to the control group. N-acetylcysteine reduced sputum from 76.9% (n = 40) of cases before the trial to 9.6% (n = 5) of cases after the trial. Spirometric components were significantly improved in N-acetylcysteine group compared to the placebo group: FEV1 (P < 0.0001), FVC (P = 0.014) and FEV1/FVC (P = 0.003). A 4-month trial with 1200 mg oral N-acetylcysteine per day can be used for treating bronchitis, but is also effective in treating bronchiolitis. It also prevents sulfur mustard-induced oxidative stress, and can be used in the treatment of sulfur mustard-induced pulmonary disease.