Pulmonary function was studied in 9000 coal miners. Flow volume curves, lung volumes, forced vital capacity (FVC), forced expiratory volume (FEV), and other indices of ventilatory capacity were measured in smoking and nonsmoking coal miners, with or without bronchitis. The average duration of work underground in miners with bronchitis for nonsmoking and smoking miners was 20.17 and 19.91, respectively; for those who did not have bronchitis, the duration was 16.18 and 17.96 for nonsmoking and smoking miners, respectively. There was a general decrease in all pulmonary function values with increasing symptoms and smoking history. FVC decreased across the group from nonsmokers with no bronchitis, to nonsmokers with bronchitis, to smokers without bronchitis, to smokers with bronchitis. The relative decrease in FEV was greater in smokers. Peak flow also decreased across the groups and showed a greater relative decrease than either FVC or FEV. The most significant difference in FEV and FVC was between smokers with bronchitis and nonsmokers without bronchitis; however, there was no significant difference between the two nonsmoking groups. Larger total lung capacity was observed in smoking than in nonsmoking miners, regardless of bronchitis incidence. This phenomenon indicated a loss of retractive force due to destruction of lung parenchyma from smoking. There was a higher, but not significant, incidence of coal miner's pneumoconiosis with increasing bronchitic symptoms and smoking history. The authors conclude that a decrease in flow at lower lung volumes is associated with cigarette smoking and small airway obstruction.