A cross-sectional study of 315 animal feed workers was undertaken in 14 animal feed mills in the Netherlands. Primary aims were to explore relationships between exposure to organic dust and respiratory symptoms and chronic lung function changes. The study comprised monitoring dust and endotoxin exposure, spirometric lung function measurements (FVC, FEV1, mean midexpiratory flow, and flow-volume parameters) and a questionnaire for respiratory symptoms. The exposure was measured in two periods, during spring and autumn. The average 8-h personal inspirable dust exposure was 9 mg/m3 grain dust (range, 0.2 to 150 mg/m3) and 25 ng/m3 endotoxin (range, 0.2 to 470 ng/m3) based on 530 personal dust measurements. On the basis of these measurements and the occupational history of the workers, the number of years "worked in dust" and an estimate of the cumulative dust and endotoxin exposure were calculated. The prevalence of most chronic respiratory symptoms tended to decrease with increasing years of exposure. The "healthy worker effect" is probably responsible for this finding. In general, a strong negative association between most of the exposure variables and lung function was found. The endotoxin exposure was more strongly related to decreases in lung function than the dust exposure. The estimated effects of an average (cumulative) endotoxin exposure on lung function were greater, with a higher statistical significance, than for an exposure to dust. These results suggest that endotoxin exposure is an important factor in the development of respiratory impairment. The lung function changes occur at endotoxin levels ranging from 0.2 to 470 ng/m3.