Preclinical studies of trans-4-[(2-amino-3,5-dibromobenzyl)-amino]-cyclohexanol hydrochloride (ambroxol, NA 872), the metabolite VIII of bromhexine, show that it is superior to the basic substance with respect to its bronchosecretolytic properties. In a double blind study 30 patients suffering from chronic obstructive bronchitis received in randomized order 36 mg bromhexine/d or 45 mg ambroxol/d, each over a period of 4 weeks. The following parameters were studied: mean bronchial flow resistance, forced expiratory volume, static lung volumes, arterial blood gases as well as clinical and laboratory results. Bromhexine was not associated with a change in lung function parameters. Ambroxol was associated with a 25% reduction of the average bronchial flow resistance. The forced expiratory volume improved over the period on the average by 14%. Patients with slight arterial hypoxemia had an increase in the partial pressure of arterial oxygen. The patients found both substances facilitated expectoration. Further studies appear necessary to confirm the improved respiratory performance under the influence of ambroxol.