A radioaerosol procedure using 99mcTc-DTPA (diethylene triamine penta acetate) was used to evaluate the permeability of the pulmonary epithelium in smokers and nonsmokers. The average clearance of this indicator from the lungs of smokers without significant airway obstruction exceeded that found in normal subjects by an average factor of more than five. This abnormality was observed throughout all lung regions. 99mTc-DTPA clearance decreased rapidly during the week after smoking was discontinued. It is concluded that smoking results in a rapidly reversible increase in pulmonary epithelial permeability.