To review studies in farming populations from temperate zones focusing on: (1) exposure to dust, bacteria, moulds, endotoxin, and ammonia, (2) sensitisation to common airborne allergens, (3) prevalence, incidence and risk factors of chronic bronchitis, asthma and bronchial hyperresponsiveness, and (4) measurements of lung function. Working in animal housings can be associated with exposure to organic dust, bacteria, moulds, endotoxin, and ammonia in concentrations that can induce cellular and immunological responses and result in respiratory diseases. Working in poultry housing might be associated with higher exposures to dust, bacteria, and ammonia than in swine and cow housings, and endotoxin exposure seems to be higher in North America than in Europe. Working exposure might influence the domestic area on farms, and there might be a protective effect of being raised on a farm regarding sensitisation and allergic diseases. Sensitisation to mites seems to be the most prevalent of the common inhalant allergens. Chronic bronchitis is frequent and data suggests that it is work related in farmers. Findings concerning asthma are less uniform, and data regarding bronchial hyperresponsiveness are too sparse and inconsistent to evaluate the effect on farming. Several risk factors have been described, and age is shared for all three clinical manifestations, while male gender, atopy, smoking, pig farming, and animal production are common risk factors for chronic bronchitis and asthma. FEV(1), and FEV(1)/FVC seems to be reduced in farmers, and longitudinal studies indicate an increased annual loss in FEV(1) in farmers, especially in pig farmers. The increased annual decline has been associated with lung function, bronchial hyperresponsiveness, smoking, automatic dry feeding systems, and endotoxin. Despite studies with methodological weaknesses, heterogenity in sampling times, measurement techniques, equipment, and diagnostic criteria, the review has revealed that the exposure to organic dust in farming can be substantial and might lead to respiratory diseases and increased annual loss in lung function. Working exposure seems to influence the domestic area in farms, and being raised on a farm might have a protective effect regarding sensitisation and allergic diseases.