A five-year follow-up of ventilatory capacity in over 95% of a random sample of men living in the Rhondda Fach has been carried out. Miners and ex-miners with and without simple pneumoconiosis have been compared with non-mining controls, and the effect of ageing, mining, dust exposure, and tobacco smoking has been assessed. The change in Indirect Maximum Breathing Capacity (I.M.B.C.) between the two surveys appeared to be independent of age, suggesting that a linear decline in this function with age is a tenable hypothesis. An average decline of 1·865 ± 0·274 litres per minute each year in the I.M.B.C. was observed, and this fall was not significantly increased either by mining or by exposure to coal dust as measured by the number of years spent working underground. In the non-miners a greater decline in I.M.B.C. was observed in smokers than in non-smokers, and this decline was greater in the heavy than in the light smokers. The decline in the non-smokers was 0·489 ± 0·714 litres per minute compared with 1·524 ± 0·319 litres per minute in the light and 3·338 ± 0·420 litres per minute in the heavy smokers. In the miners and ex-miners without pneumoconiosis a greater decline in I.M.B.C. was observed in smokers than in non-smokers, but there was no relation between the rate of decline and the amount smoked. The decline in the non-smokers was 0·950 ± 1·071 compared with 2·164 ± 0·485 and 2·080 ± 0·428 litres per minute in the two smoking groups. In the men with simple pneumoconiosis (category 3) there appeared to be no relation between decline in lung function and smoking habits. The decline in non-smokers was 1·492 ± 0·594 compared with 1·956 ± 0·357 and 1·438 ± 0·467 in the two smoking groups. The decline in I.M.B.C. over five years in non-miners smoking 15 g. tobacco per day and over was significantly greater than the fall estimated from the age specific trend. A possible explanation is that there has been a recent change in the effect of heavy smoking on ventilatory function resulting in a more rapid decline. A greater decline in I.M.B.C. was observed in men with respiratory symptoms than in those without.