Five hundred and twenty-six patients suffering from chronic bronchitis were randomized to receive either N-acetylcysteine (NAC) or placebo during a 6-month period. The aim was to compare the number of acute exacerbations in the two groups. General practitioners were asked to enter patients with a diagnosis of chronic bronchitis, based on the MRC criteria. We failed to find any statistically significant difference in the number of exacerbations between the two treatment groups although there was a slight trend towards improvement in the NAC group during the first 3 months of the trial. The tolerability was similar for both treatments. Patients taking NAC showed a reduction in number of days on which they were incapacitated and this result was statistically significant.