Abstract: Apparatus for measuring airway resistance, which apparatus comprises a housing, a bore in the housing, a flow measuring device which is positioned in the bore and which is for measuring a flow of air through the bore caused by a person exhaling or inhaling, an interrupting valve device which is for interrupting the flow of air through the bore, a drive device for moving the interrupting valve device between a bore closing position and a bore opening position, and a pressure sensor device for sensing variations in pressure in the bore caused by the interrupting valve device closing and opening the bore and thus interrupting the flow of air through the bore, and the apparatus being such that the bore is of circular cross section, and the interrupting valve device is a non-circular eliptical plate member which is mounted in the bore for pivotal movement by the drive device between the bore closing position and the bore opening position and which extends at an acute angle to the bore in the bore closing position.