Indices of forced expiration (FEV1 and MEF25-75) were compared with the slope of Phase III of the single breath nitrogen test (%N2/1) in 1270 men, who, based on life occupational histories, were categorized as cement factory workers, blue or white collar workers, and farmers. The slope of Phase III was successfully determined in 1182 men. Irrespective of occupational category, the FEV1 and MEF was lower in present smokers than in ex-smokers, who in turn had lower values than never-smokers. With corresponding smoking habits, white collar workers showed on average higher values of FEV1 and MEF than the blue collar workers, the cement-exposed men and the farm workers. The slope of Phase III varied with smoking habits in a similar way, and among present smokers, the same occupational gradient was evident. However, no effect of occupation on the slope of Phase III could be traced in ex-smokers and never-smokers. It is concluded that the response of the lung to occupational exposure may differ from that of tobacco smoking. The combined use of indices from the forced expirogram and the slope of Phase III could yield valuable information in the study of occupational respiratory diseases.