Pulmonary diffusing capacity (DL) was measured in 133 underground bituminous coal miners at rest and during moderate exercise by the steady state end tidal sampling method. The average DL. among cigarette smoking miners was lower than among nonsmoking miners of comparable age and years of underground mining. In contrast to the smoking miners, a large majority of nonsmoking miners had normal DL. This difference in DL could not be explained by the presence of bronchitis, airway obstruction, or radiographic evidence of coal workers' pneumoconiosis (CWP), although bronchitis and CWP were more frequent among the smoking miners. It is doubtful that the measurement of DL would be a useful screening test for occupational disease among cigarette smoking coal miners, since it is significantly affected by smoking.