This study was based on data provided by 9,483 farmers from a larger sample of 12,056 Finnish farmers. The data were collected in two postal surveys, with a 3-year interval between them, conducted by the Social Insurance Institution of Finland. About one-third of the whole population were smokers. Among men the prevalence of smoking was 58.6% and among women 10.2%. Among men the prevalence of smokers was larger in older groups, among women the situation was reversed. Based on past or present infantile eczema, atopic dermatitis, or allergic rhinitis, one-third of the subjects were classified as atopic. The prevalence of atopy was essentially the same in all age groups. The prevalence of chronic bronchitis was 8.0%, and the mean annual incidence was 2,017 per 100,000 farmers. Among farmers who tended cattle the prevalence of farmer's lung was 1.7% and the mean annual incidence was 540 per 100,000 similar farmers. Chronic bronchitis was twice as common among atopic as among non-atopic subjects and twice as common among smokers as among non-smokers. Atopy and smoking seemed to have an additive effect on both the prevalence and incidence of chronic bronchitis. The results strongly support the so-called "Dutch hypothesis" about the natural history of chronic bronchitis, according to which chronic bronchitis is closely related to asthma. Farmer's lung was only slightly more common among atopic than among non-atopic subjects and twice as common among non-smokers as among smokers.