On 17 ex-smokers (7 without chronic bronchitis, 10 with chronic bronchitis) and 48 smokers (13 without chronic bronchitis, 35 with chronic bronchitis) we analyzed the correlation between smoking habits (pack years, smoking and ex-smoking time), mucociliary clearance rate (mC) and airways obstruction. The mC was measured with 99mTc tagged monodisperse erythrocytes. The static and dynamic lung volumes, the maximal expiratory flow volume curve and the airway resistance were measured by whole body plethysmography. The ex-smokers without chronic bronchitis showed the same mucociliary clearance rate (t-mC in 1 h = 38.3 +/- 10.3%) as the 80 control subjects who never smoked (t-mC in 1 h = 36.9 +/- 12.6%). But the subjects who never smoked showed less airway obstruction. The mucociliary clearance rate in normal subjects who never smoked is a function of age: t-mC in 1 h = -0.37 X age + 53; c-mC in 1 h = -0.45 X age + 73; p-mC in 1 h = -0.25 X age + 38. The smokers without chronic bronchitis showed normal ventilatory function tests but a lower mC rate (t-mC in 1 h = 27.8 +/- 12.3%) than the healthy ex-smokers and the control subjects who never smoked (p less than 0.01). The ex-smokers and smokers with chronic bronchitis had a lower mC rate (t-mC in 1 h = 21.2 +/- 11.3% and 18.1 +/- 9.1) and more airway obstruction (p less than 0.001) than the subjects who did not report any symptoms of chronic bronchitis. The ex-smokers with persistent symptoms of chronic bronchitis showed the severest degree of airways obstruction. The smokers with chronic bronchitis showed the most delayed central mC (c-mC) rate, but less airway obstruction than the ex-smokers with persistent symptoms of chronic bronchitis. The smoking habits (pack years) correlated with the decreased mC rate (p less than 0.01) and the degree of airways obstruction (p less than 0.001).