A hand-held instrument designed to measure only the maximum rate of expiratory gas flow [peak expiratory flow (PEF) or peak expiratory flow rate (PEFR)] from the lungs. It is in the form of a cylinder with a mouthpiece for exhalation, and a calibrated analogue scale that shows the peak flow value. The device helps to assess pulmonary function in routine tests performed in or outside a clinical setting; it is also intended for periodic self-evaluation of the respiratory status of a patient, and to help in the treatment evaluation of patients affected by chronic respiratory disorders (e.g., asthma, emphysema). This is a reusable device.