In 1977 an epidemiological survey was carried out on incidence of chronic bronchitis in 652 workers (age 20-64) of a coking plant. The survey was repeated six years later in 300 of the original participants (56%). The aim of the study was to assess the effect of smoking, age and occupation on incidence of chronic bronchitis and dynamic change of FEV1 values during the six years. During the 6 years the incidence of chronic bronchitis rose from 22% to 43%. Mean annual decrease of FEV1 was higher in smokers as compared to nonsmokers (57.4 versus 52.6 ml per year). Higher mean falls of FEV1 values were noted in subjects with occupational risk factors present (68.4 ml) in comparison with those with low occupational exposure (51.2 ml). Susceptible subjects presented a thrice higher annual FEV1 decrease rate (142 ml).