Airway mucus hypersecretion and increased oxidative stress are clinical and pathophysiological features of asthma exacerbation. We studied effects of N-acetylcysteine (NAC) as a mucolytic and antioxidant agent in asthma exacerbation. In this randomized, single-blinded, placebo-controlled study 50 patients ( 17 male, 33 female, mean age 48.94+/-13.68) with asthma exacerbation were randomized to receive either oral 600 mg b.d. N-acetylcysteine or placebo in addition to standard treatment during 5 days hospitalization. Daily measurements of wheezing, dyspnea, cough, sputum, expectoration, night sleep scores and morning PEFR were performed. There was no significant difference in wheezing score between patients assigned NAC and those assigned placebo in day 5(0.84[SD 0.94] VS 0.87[SD 0.79]) and also in cough score (0.72[SD 0.84] VS 0.79[SD 0.97]), dyspnea score (0.84[SD 1.06] VS 0.91[SD 1.01]), sputum score(0.79[SD 0.83] VS 0.62[SD 0.71]), expectoration score(0.79[SD 0.97] VS 0.83[SD 1.09]), night sleep score(1[SD 1.17] VS 0.67[SD 0.98] and morning PEFR (256[SD 96.36] VS 282[SD 98.86]). We concluded that addition of N-acetylcysteine to usual asthma medication has no significant effect in treatment of asthma exacerbation.