We have assessed lung clearance of inhaled radioactive particles, predominantly deposited in the smaller bronchi, in 60 tests on patients with chronic bronchitis. When tests were matched for sputum production, clearance in ex-smokers was not significantly faster than in patients still currently smoking. Particle clearance also did not depend on FEV1 or--within the range studied--on the depth of particle deposition. Clearance was significantly faster in the patients who coughed frequently and produced high volume of sputum. For some patients with chronic obstructive bronchitis, giving up smoking may lead to poorer mucus clearance because of decreased transport by productive cough.