The effect of coal dust on 9,076 coal miners was investigated over a 10 year period. Thirty one mines in ten states were selected because they represented different coal seams and mining methods. Each miner was given a test for forced ventilatory capacity (FVC), total lung capacity, forced expiratory volume (FEV), roentgenograms, and the standard anteroposterior examination. Since there were no comparisons, the observed pulmonary functions were compared to predicted values. There was a greater disparity between the observed and predicted FEV and FVC for anthracite than bituminous miners, while a comparison of the observed and predicted pulmonary function values according to geographic areas showed lower observed mean FEV than predicted FEV. The mean FVC for the regions resembled each other closely with the exception of the anthracite region. These regional variations were not related to differing smoking habits. In miners with simple pneumoconiosis, no relation was found between the various indices of pulmonary function and radiographic category. However, complicated pneumoconiosis was definitely associated with ventilatory impairment. The residual volume showed a slight increase with increasing radiographic category of simple pneumoconiosis for all miners from all regions. The significant geographic variations in FVC and FEV appeared to be related to the type of coal dust to which the miners were exposed, to the ethnic origin of the miners, and to other miscellaneous nonoccupational factors. The authors conclude that while coal mining may result in minor reduction of ventilatory capacity, these reductions would have minimal effect in the absence of complicated pneumoconiosis, and would not cause respiratory disability.