Chronic bronchitis has a prevalence of approximately 11% in the population aged over 35 years and its frequent acute exacerbations (AECBs) are an important cause of morbidity and costs in health-care resources. Oral N -acetylcysteine (NAC) is administered during the winter months as a way of reducing AECBs. This cost-effectiveness analysis was done from the payers' point of view in the Swiss health-care system, based on a retrospective analysis of published placebo-controlled studies. The pooled data show that continuous administration of 400 mg day(-1)per os of NAC leads to a significant reduction in the number of AECBs (NAC: 16.2 vs 25.2% AECBs per month); a significantly smaller percentage of days of sick leave (NAC: 3.6 vs 5.3%) and a lower rate of hospitalizations (NAC: 1.5 vs 3.5% over a period of 6 months). Taking into account the poor compliance of these patients, calculations assumed a compliance of 80%. Direct costs were those of an NAC treatment, the management of an AECB (biological tests in 59%, X-rays in 65% and pulmonary function tests in 45%; antibiotics 70%, bronchodilators in 89%, corticosteroids in 24% and 'others' in 25% of the patients), and of hospitalizations (estimated at 10 days per case). Based on these figures, the mean direct costs of an untreated patient were CHF 869 vs CHF 700 in the NAC-treated patient. Univariate sensitivity analysis indicated that cost neutrality is reached with 0.6 (<0.25-1. 94, 95% CI) AECBs per 6 months. Indirect costs (based on sick leave) were also significantly different; the mean in untreated patients was CHF 1324 vs CHF 779 in the NAC-treated patients. Conclusion: Treating chronic bronchitis patients with NAC during the winter months is cost-effective both from the payer's and a social point of view.