Mucosolvan is a 'mucus-modifier' whose active compound is ambroxol. The aim of our research was to determine its clinical efficacy in the treatment of bronchial stasis by carrying out two successive double-blind studies in two parallel groups of patients and comparing ambroxol with placebo in a treatment lasting 10 days. The first trial involving 60 patients daily treated with 120 mg of ambroxol showed significant differences (p less than 0.05) in sputum volume, sputum viscosity, difficulty of expectoration, and severity of cough when compared with the placebo group. The two groups of patients were initially well matched and the drug was well tolerated. In a second trial involving 60 new patients daily treated with 30 mg of ambroxol, the drug showed no clear clinical effects. We conclude that ambroxol is an effective 'mucus modifier' and is well tolerated at a dose of 120 mg/day.