The rheological properties of bronchial mucus samples, collected from randomly selected patients with chronic bronchitis by protected expectoration, under steady-state conditions without any exacerbation, were investigated in a double-blind multicentre study before and after five days of treatment with 4.5 g/day carbocysteine or with glucose as a placebo. Viscous and elastic properties of the mucus were measured with a rheometer fitted with coaxial cylinders set up in an oscillating instead of a rotating mode. The shapes of the ellipses obtained characterized the rheological properties of each bronchial mucus sample before and after treatment. Two different rheological patterns were observed. In the group of patients with initial viscosity greater than or equal to 10,000 mPa.s-1, carbocysteine treatment reduced viscosity and elasticity more than those of the placebo-treated patients. In the group of patients with viscosity lower than 10,000 mPa.s-1, the rheological modifications were the same for both groups. These results are discussed in terms of both the efficacy of carbocysteine and the necessity of rheological characterization of the patients before treatment into different groups, according to the rheological properties of their secretions, for better and targetted therapy with mucus modifying drugs.