Erdosteine is a new thiol compound with effects on bacterial adhesiveness as well as antioxidant and mucoactive properties. The EQUALIFE study, a fully randomized, double-blind, placebo-controlled, parallel-group, multicenter study, was designed to assets the effectiveness of long-term treatment with erdosteine in patients with moderate chronic obstructive pulmonary disease (COPD). One hundred and fifty-five patients received oral erdosteine, 300 mg b.i.d., or placebo for 8 months during the winter season to assess the effect of treatments on exacerbation rate, hospitalization, lung function and quality of life, assessed using the Short Form 36 and the St. George's Respiratory Questionnaire. A pharmacoeconomic analysis was also conducted to compare the two treatments. One hundred and twenty-four patients completed the study with erdosteine (n = 63) or placebo (n = 61). The group of COPD patients who received 8 months of continuous treatment with erdosteine had significantly fewer exacerbations and spent fewer days in the hospital than did the placebo group; furthermore, they had no loss of lung function. Patients in the erdosteine group also showed a significant improvement in health-related quality of life. The mean total COPD-related disease costs per patient were lower in the erdosteine group than in the placebo group over the study period. The results indicate that 8 months of treatment with erdosteine is effective in reducing exacerbation and hospitalization rates and in improving health status. The study suggests that erdosteine is likely to provide an important contribution to the therapy of patients with symptomatic COPD.