A study of longitudinal changes in ventilatory function in United States (US) coal miners was conducted. Data collected in the National Coal Study which was an ongoing investigation of lung disease in workers in 31 US coal mines and its association with dust exposure were utilized. Data on forced vital capacity, 1 second forced expiratory volume (FEV1), smoking, occupational history, respiratory symptoms, and mine dust exposures obtained from the Mine Safety and Health Administration collected on 1072 coal miners in surveys conducted from 1969 to 1971 and from 1977 to 1981 were analyzed for possible associations. The mean age of the subjects at the midpoint between the two surveys was 44 years. The miners had spent a mean of 11 years underground before the 1969 to 1971 survey, and about 80% of the time underground between the two surveys. Overall dust exposures in the mines averaged 1.2mg/m3. Approximately 40% of the miners were current smokers. The miners showed an overall mean decrement in FEV1 of 0.48 liter (l) between the two surveys. The FEV1 decrements increased with age, height, and smoking. Regression models based on years of work underground, years of work at coal faces, coal dust exposure, and prior underground work indicated that smoking was associated with an excess FEV1 decrement of 0.10l. Employment as a coal miner or exposure to coal dust was associated with FEV1 decrements of 0.036 to 0.084l. The data confirmed the relationship between work in coal mines and loss of ventilatory function seen previously in studies of British coal miners. The decrease in ventilatory function was more pronounced in smoking miners.