Abstract: A peak flow meter has a flexible resilient vane which is rigidly mounted by one end and which has its other end engaged with a slider indicator which slides in a slot formed in a curved wall of the casing. In use, the human subject blows into the casing through a mouthpiece. The vane is bent by the expiration which vents through the part of the slot between the slider and the mouthpiece. The slider stops at the furthest point from the mouthpiece to which it is moved by the vane and that point is an indication of the peak expiratory flow of the human subject.