The mucociliary clearance characteristics and the correlation between the extent of smoking and bronchial mucociliary dysfunction were studied in 71 smokers (15 without chronic bronchitis, 16 with simple and 40 with obstructive chronic bronchitis). Mucociliary clearance (MC) was measured with 99mTc-tagged human erythrocytes. Lung function values were determined by whole body plethysmography. All three smoking groups had significantly (p less than 0.001) lower TMC60.min rates (total MC in 60 min) on the average than the predicted values. The mean TMC60.min rates of heavy smokers with simple and obstructive chronic bronchitis were significantly (p less than 0.001) lower than that of light smokers without chronic bronchitis. There was no significant difference between the TMC60.min rates of smokers with simple and obstructive chronic bronchitis. The decrease in the MC rates was more pronounced in the central than in the peripheral airways. A significant (p less than 0.001) correlation was found between the extent of smoking (pack years) and the decrease of MC. The results suggest that the decrease of MC is an early functional abnormality in smokers, which precedes the development of symptoms of chronic bronchitis as well as functional detectability of airways obstruction.