The flow-resistive characteristics of a variety of commercially available expiratory positive-pressure valve systems used to provide continuous positive airway pressure (CPAP) and positive end-expiratory pressure were evaluated. One flow-resistor and seven threshold-resistor expiratory pressure valve systems were set at 5, 10, 15, 20, and 25 cm H2O of expiratory pressure, and sinusoidal exhaled flows peaking at 50,100, and 200 L/min were directed through each valve at each level of expiratory pressure. The Siemens flow-resistor valve demonstrated the greatest deviation in pressure above set CPAP levels at peak flow rates of 100 and 200 L/min, which suggests high resistance to exhaled flow. The Vital Signs threshold-resistor valve demonstrated the least deviation in pressure from set CPAP levels at all rates of exhaled flow, which suggests low flow resistance. The Emerson and IMV Bird threshold-resistor systems resisted flow less than the BEAR-2 and the Puritan-Bennett MA-2 and 7200 inflatable-balloon threshold-resistor-like valve systems. These data suggest that threshold resistors may be classified as low-resistance or high-resistance types. Using only low-resistance threshold resistors for CPAP may minimize the incidence of barotrauma and other deleterious effects related to airway pressure.