Mucus hypersecretion is a hallmark of chronic obstructive pulmonary disease (COPD) and is regarded as contributing to airflow limitation in these patients. The value of mucolytic therapy is still a matter for debate. We investigated the short-term influence of an inhalative mucolytic drug mercapto-ethane sulfonate (Mesna) on the lung function, respiratory mechanics and viscous respiratory load in patients with COPD in a randomized, placebo controlled, double-blind, cross-over study in 10 patients with moderate, stable COPD. Resistance, static compliance and frequency dependent dynamic compliance and viscous respiratory work, FVC, FEV(1), PEFR, MEF(75), MEF(50), MEF(25), TLC and RV were measured. These lung function parameters did not show any significant differences between the treatment groups. In conclusion, an inhalative treatment with an active mucolytic drug of patients with mild COPD does not improve respiratory mechanics and respiratory load.