It is important to find interventions that will reduce the frequency and severity of exacerbations of COPD, because of their effect on morbidity and healthcare expenditure. A Cochrane systematic review included 23 studies that had evaluated the effects of treatment with mucolytic agents in patients with chronic bronchitis or COPD. Mucolytic treatment was associated with a significant reduction of 0.79 exacerbations per patient per year compared with placebo, a 29% decrease. Patients who received treatment with mucolytic agents were twice as likely to remain exacerbation-free in the study period than if they had received placebo, with six patients needing regular treatment with mucolytic agents for 3-6 months to achieve one less exacerbation over that time. Treatment with mucolytic agents resulted in nearly 7 days less illness per patient per year. How mucolytic agents work is unknown, although they may reduce exacerbations by altering mucus production, antioxidation, or antibacterial or immunostimulatory effects. They do not appear to affect the decline in lung function that occurs in COPD. The treatment appears to be without any adverse effects, apart from the need to take oral medication daily. Cost-effectiveness analysis suggests that the point at which the costs of treatment and non-treatment were equal was 1.2 less exacerbations per year. This is higher than the effect observed in the Cochrane review, suggesting that treating everyone with COPD with mucolytic agents would not be cost effective. Those with more frequent and severe exacerbations appear to have the most to gain.