A follow-up was carried out on 21,579 male mass radiography volunteers aged at least 40 who had been the subject of an earlier investigation in which their smoking habits and sputum production were recorded and the prevalence of lung cancer was determined after chest x-ray examination. During the follow-up period, which was a minimum of 36 months and a maximum of 56 months, 64 new cases of lung cancer were identified by cross checking records with the registers of the regional cancer registration bureau. A significantly higher incidence of lung cancer was found in those with chronic bronchitis than in those without this disease. In the smoking categories, cigarette smokers with chronic bronchitis had a higher incidence than those without it, and this relationship was maintained irrespective of age and amount smoked. It is concluded that persons who smoke run a higher risk of chronic bronchitis than non-smokers and those who develop bronchitis run a higher risk of developing lung cancer.