Abstract: Peak expiratory flow meter for measuring the expiratory flow rate of a subject having a flow inhibitory device normally effective to inhibit expiratory flow through the peak expiratory flow meter, the flow inhibitory device being operable in response to attainment of a predetermined expiratory pressure to allow the expiratory flow. From the PEF measured, the airway passage resistance can be determined. Further lung characteristics can be determined from the shape of the decay portion or the expiratory flow curve. The meter has clinical application in the monitoring and diagnosis of asthma and other pulmonary diseases.