We propose a forced oscillation test modality for detecting upper airway obstruction (UAO) as an alternative to spirometric UAO indices in patients with tracheal stenosis. From oscillometry performed at different breathing flow rates, airway resistance at 5 Hz was determined at 0.5 l/s (R), and flow dependence of resistance was computed as the regression slope of resistance vs. flow up to 1 l/s (Delta R/Delta V). It was first verified by measurement in 10 normal subjects and 10 patients with chronic obstructive pulmonary disease that Delta R/Delta V was unaffected by the presence of peripheral airway obstruction and that external orifices (with lumen area down to 28 mm(2)) induced marked increases in R and Delta R/Delta V. Ten patients eligible for tracheal dilatation underwent spirometry and impulse oscillometry before and after intervention. Considering the lumen area of tracheal stenosis in the patients (42 +/- 28 mm(2), represented as the mean +/- SD), the R and Delta R/Delta V increases were of similar magnitude to those predicted by the external orifices. In addition, R (r = -0.68; P = 0.001) and Delta R/Delta V (r = -0.65; P = 0.001) showed better correlations with minimal tracheal lumen than any spirometric UAO index. Delta R/Delta V, but not R, showed a consistent return to normal after intervention in the stenosis patients. We conclude that the forced oscillation test at different breathing flow rates up to 1 l/s provides a measure of UAO, namely flow dependence of resistance Delta R/Delta V, which can signal a critical level of tracheal stenosis and is not confounded by the presence of concomitant peripheral airway obstruction.