Valves for positive expiratory pressure (PEP) can be characterized as threshold resistors, ideally providing pressure independent of the expiratory flow, or as flow-dependent resistors. The aim of the study was to evaluate the flow-dependence properties of PEP devices of the flow resistor type compared to threshold resistor devices. Pressures were measured on three different flow resistor valves: the PEP-mask, the Pari-PEP-System and the System 22-PEP with orifice diameters of 1.5-5.0 mm; and on three threshold resistors, the underwater seal, the Ambu Positive End-Expiratory Pressure (PEEP) valve and the Vital Signs PEEP valve with pressures of 0, 5, 10, 15 and 20 cmH2O. All devices were studied with constant flows of 3, 6, 9, 12, 15, 18, 30, 60, 90, 120 and 150 l.min-1. The PEP-mask, the Pari-PEP and the System 22-PEP showed the typical pattern of flow resistors, i.e. a pressure increasing with flow, dependent on the diameter of the orifice. The underwater seal and the Vital Signs PEEP valves acted as almost ideal threshold resistors. The Ambu PEEP valves acted as threshold resistors at the lower flows, but showed flow-dependency at higher flows. The Vital Signs PEEP valves gave lower pressures and Ambu PEEP valves gave higher pressures compared with indicated values, whereas the underwater seal gave the intended pressure. In clinical use of PEP treatment the actual pressure should be measured to ensure the intended pressure, no matter which type of resistor is used.