This is a randonmized, masked, placebo-controlled parallel group, clinical trial to evaluate the effects of three different doses of N-acetyl L-cysteine (NAC) and placebo on patient safety and on physical and transport properties of expectorated sputum. Patient sumptoms, quality of life and exacerbation will also be followed as well as pulmonary function testing and functional exercise capacity. We hypothesized that a prolongeed course of oral NAC favorably affects the morbidity of chronic bronchitis, particularly the incidence of acute exacerbations.