The aim of this study was to determine the influence of smoking habit and occupational exposure to air pollution on the prevalence of chronic bronchitis and lung function efficiency in foundry workers. MRC survey and spirographic measurements were performed in 917 men aged 18-62 years (mean age 38.6). In 455 men (49.6%), occupational exposure to air pollution significantly exceeded the permissible values. In 122 men (13.3%) the exposure was low. Chronic bronchitis was more than twice as frequent in smokers (27.3%) as in nonsmokers (11.0). Cessation of smoking led to an improvement in the symptoms of chronic bronchitis. Chronic bronchitis was slightly more frequent in highly exposed (25.5%) as compared with low-exposed workers (15.6%). Age-related lung function decline was significantly greater in smokers than in nonsmokers. Unlike chronic bronchitis, lung function was still lower after smoking cessation. Among highly exposed workers, age-related FVC and FEV1 declines were significantly larger in smokers than in non-smokers.