Over the past ten years we have performed 47 tracheoplasty for non-neoplastic disease. Of the 47 patients, 17 had post-tracheostomy stenosis and 17 tracheobronchial tuberculosis. Non-neoplastic disease causes stenosis in the trachea to various degrees. Provided the lung function of these patients is not reduced, tracheoplasty may not be necessary. Functional diagnosis of the tracheal stenosis is therefore important in determining the indications for tracheoplasty in this condition. Clinically, evaluation of the flow plateau of the flow-volume curve has been utilized as a method for detection of fixed upper airway obstruction and the ratio of FEV1.0 to PEFR is helpful for an assessment of the degree of stenosis. But it has not been explained why the ratio correlates with the degree of stenosis. In this study, fixed-type tracheal stenosis was simulated in seven normal subjects by the use of a mouthpiece with an orifice of variable diameter. We derived the experimental formulae of FEV1.0/PEFR as follows. FEV1.0/Qp = FVC/Qp-e2(FVC/Qp-1.5)/2 Qp = 48.7 alpha 1.12 (1-1.1 alpha) (Qp; the plateau flow of the flow-volume curve) (alpha; the ratio of the cross-sectional area of the stenotic portion to that of normal trachea) The values of FEV1.0/Qp calculated from FVC and alpha using these formulae were equal to or in the neighborhood of the experimental values. On the basis of these calculations we concluded that the ratio FEV1.0/PEFR is helpful in the assessment of the degree of stenosis.(ABSTRACT TRUNCATED AT 250 WORDS)