Abstract: The air diffuser which is designed to be attached to an air supply duct comprises a cannister having a closed and an air inlet pipe proximate its closed end for attachment to the air supply duct. The cannister also includes attachment means proximate its open end so that the cannister can be secured to a supporting frame. A hollow sleeve having an open bottom end is slidingly insertable into the open end of the cannister, and at its top end is provided with a first array of air discharge ports. Co-operating means are located on an interior sidewall of the cannister and an exterior sidewall of the sleeve which permits the sleeve and the cannister to be telescoped together in a non-rotational relationship and which also enables the sleeve to be detachably connected to the cannister at selected telescopic positions.