Summary:A double‐blind crossover clinical trial designed to evaluate the mucolytic activity of bromhexine hydrochloride on sputum was completed by 15 patients with chronic lung disease. All the subjects chosen were troubled by production of tenacious mucoid or mucopurulent sputum at the beginning of the trial. Each subject received 16 mg of bromhexine by mouth, three times daily and identical appearance placebo tablets, for an equal number of weeks, in random sequence.Twenty‐four‐hour sputum production was collected for each subject throughout the trial. Viscosity of sputum was evaluated with a Wells‐Brookfield Micro‐Viscometer Model RVT, studying the highest viscosity sample produced per 24‐hour period. In eight patients some significant decrease in sputum viscosity was observed during bromhexine therapy. In one patient a significant decrease in sputum viscosity was observed while he was receiving the placebo. The mean sputum viscosity for all 15 patients considered as a group was significantly lower during bromhexine therapy than during the placebo therapy.It is concluded from the results obtained that bromhexine has a demonstrable mucolytic effect on tenacious sputum and consequently may play a useful role in the management of patients with chronic lung disease involving the production of high viscosity bronchial