We measured the flow-resistance of five commercially available 10 cm H2O expiratory positive-pressure (EPP) valves (n = five per valve type) at bias flows of between 0 and 2,000 ml/s. We found that individual valves of each type and manufacturer functioned similarly. Different valve types, however, functioned differently: with one type, system pressure was higher than rated (p less than 0.05), and with another type, system pressure was significantly flow-dependent (p less than 0.01). The remaining types of valves had no flow-resistive properties and maintained a system pressure of 10 cmH2O. We conclude that system pressure is not similar in all continuous positive airway pressure (CPAP) systems using bias flow and EPP valves. The work of breathing imposed by CPAP circuits will be increased in systems whose EPP valves have flow-dependent properties.