The prevalence of chronic bronchitis and of clinical farmer's lung was studied in 30 districts of the French Doubs province in relation to individual (age, sex, smoking) and geographical (altitude) factors. 5703 exclusively dairy farmers (response rate 83%) participated in the study by answering a medical questionnaire. Prevalences of chronic bronchitis and clinical farmer's lung were 9.3% and 1.4% respectively. A logistic regression model was used to evaluate risk factors for chronic bronchitis and clinical farmer's lung. A risk of chronic bronchitis was associated with male sex (p < 10(-4)), age (p < 10(-4)), smoker category (p < 10(-4)), and altitude (p < 10(-4)). A risk of clinical farmer's lung was associated with non-smokers (p < 0.05), and linearly with altitude (p < 10(-4)). Also there was a strong positive relation between chronic bronchitis and clinical farmer's lung (odds ratio 19.5 (95% confidence interval 12.1-31.4) after adjustment for confounding variables. The main finding of this study is the highly significant increase of prevalence of the diseases in relation to altitude.