Miniature peak flow meter is compact, portable diagnostic device of beeper size. It consists of two bodies which create a curved cylinder inside which moves a rotational piston by means of rotation. The resistance to that movement is achieved by a specially created torsion spring. The mouthpiece is folded inside the device in unused position thus contributing to the compactness. By pulling out the mouthpiece and blowing through it, the patient rotates the piston by his expiratory flow and moves a pointer secured in a groove on the top side of the device. The pointer shows the value on a printed scale, and the patient writes it on a recording chart placed on the bottom surface of the device.