Few data are available on the relative sensitivity of different lung function tests in upper airway obstruction (UAO) and on the correlation of these tests with radiological estimations of tracheal stenosis. This paper is based on patients with a goiter or tracheal tumor and in healthy subjects breathing through rigid resistors, as well as on literature data. Lung function abnormalities present a specific pattern in UAO: this is characterised by a typical shape of the maximal flow-volume curve, a marked reduction in peakflow (PEF) and only minor changes in routine spirometry (e.g. FEV1, which thus tends to underestimate the stenosis). The clinical relevance of the lung function abnormalities in UAO can be estimated from their relationship with the exercise limitation: a moderate exercise limitation to about 60% pred is found if PEF is reduced to about 40% pred, and Raw is increased to about 600% pred (in which instance FEV1 is still 75% pred). This limitation grossly corresponds to a UAO of about 6 mm diameter. No good correlation is found between lung function abnormalities and the radiological stenosis (CT-scan or X-ray): CT-scan or X-ray are only moderately correlated with each other. Moreover, stenosis can be visualised through X-ray in only about 60% of the cases.