Acute exposures to hemp dust, in healthy subjects as well as hemp workers with byssinosis, resulted in two different responses. Men with symptoms (chest tightness, coughing, and wheezing) after exposure showed decreases of forced expiratory volumes (FEV(1.0)), flow rates on maximum expiratory flow-volume (MEFV) curves, and of vital capacity (VC), while airway conductance (Gaw: TGV ratio) did not decrease significantly ("flow rate response"). Men without symptoms after exposure showed no changes of VC, FEV(1.0), and MEFV curves, but had a significantly decreased airway conductance ("conductance response"). The flow rate response is attributed to a pharmacological bronchoconstrictor effect of hemp dust on small airways, the conductance response to a mechanical or reflex effect of hemp dust on large airways. Both responses were abolished by a bronchodilator drug. The type of response reflects a difference between individuals and is not related to age, smoking habits, or prior exposure history. Men with normal control function data had either a flow rate or a conductance response. All men with abnormal control data had a flow rate response.Long-term hemp dust exposure causes irreversible obstructive lung disease, in particular among men who respond to acute dust exposure with symptoms and flow rate decreases. The detection of this response, with FEV(1.0) measurements and MEFV curves, is essential in the study of byssinosis. Decreases of airway conductance after dust exposure have no consistent relation to the development of clinical symptoms. The relative value of measurements of maximum expiratory flow rates and of airway conductance in other lung diseases needs to be reassessed.