Chronic obstructive pulmonary disease (COPD) is an irreversible disorder characterized by airflow obstruction and a progressive decline in forced expiratory volume in one second (FEV1). At present, no treatment except quitting smoking appears to affect the progression of the disease. Oxidative stress has been implicated in its pathogenesis. The Bronchitis Randomized on NAC Cost-Utility Study (BRONCUS) is a phase III, randomized, double-blind, placebo-controlled, parallel group, multicentre study designed to assess the effectiveness of the antioxidant agent N-acetylcysteine (NAC) in altering the decline in FEV1, exacerbation rate, and quality of life in patients with moderate to severe COPD. In addition, cost-utility of the treatment will be estimated. Patients will be followed for 3 yrs and evaluated every 3 months. The necessary sample size to demonstrate an effect on the decline in FEV1 of 20 mL x yr(-1) was estimated to be 478 patients. Five hundred and twenty-three patients with moderate to severe COPD were recruited from 10 European countries from June 1, 1997-December 31, 1999. They were 63+/-8 yrs old and consisted of 243 (46%) current smokers and 280 (54%) exsmokers. Patients had on the average 4.9+/-1.6 exacerbations during the last 2 yrs. Postbronchodilator FEVI averaged 57+/-9% and the reversibility after 400 microg of Salbutamol averaged 4+/-4% predicted. The final results of the trial will be available in about 2 yrs. The study will provide objective data on the effects of N-acetylcysteine on outcome variables in chronic obstructive pulmonary disease.