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Backgrounds: The measurement of volume of isoflow has been considered as a sensitive test for detecting small airway diseases showing normal pulmonary function in a routine pulmonary function test. To evaluate the functions of small airway among dust exposed workers, the changes of volume of isoflow were measured and its applicability of managing early stage pneumoconiosis patients was studied. Method: The subjects were 67 male, pneumoconiosis with small opacity and FEV1>80%, FEV1/FVC>75% in spirometry and the controls were 20 male, no dust-exposed office workers. The maximal epiratory volume curves after inhalation of indoor air and gas mixtures were measured and between the dust exposed and control workers were compaired. Results: 1) There were no significant differences between two group in and . But the ratio of of the subjects was siginificantly higher than that of the control (p is a very useful index in early detection of small airway dysfunction. 2) The ratio of of the subjects was signigicantly different between only smoker group and mixed group(smoker and nonsmoker). It suggestes that smoking is an important cousative factor of small airway dysfunction. 3) As the profusion of the chest X-ray increased, the rartio of increased, but no significant difference of was found between categories of pneumoconiosis. The categories of pneumoconiosis and small airway dysfunction may not be related. 4) No significant relationship was established between the duration of work and the ratio of . Conclusions : It is concluded that the measurement of is useful to detect small airway dysfuction of early stage pnuemoconiosis patents with small opacities but showing normal pulmonary function in a routine pulmonary function test.