BACKGROUND: Continuous positive airway pressure (CPAP) maintains a constant pressure to reduce the patient's work of breathing (WOB). The aim of this study was to measure the additional WOB imposed by four current CPAP devices during simulation of a difficult but commonly encountered clinical situation. METHOD: Flow contour, respiratory system compliance and total lung-airway resistance of a patient under CPAP were simulated. The devices were tested at a CPAP of 15cm H(2)O with a heated humidifier and a nasal pillow, which increased circuitry resistance and with and without a simulated unintentional leak. RESULTS: With no leak, positive end-expiratory pressure (PEEP) at the interface varied across devices from 14.0 to 15.3cm H(2)O. With a leak of 1L/s, PEEP varied from 11.5 to 17.1cm H(2)O. Imposed inspiratory WOB ranged from less than 0.1J/min to 0.45J/min with no leak, and the range broadened with leaking. Findings were similar for the imposed expiratory WOB. CONCLUSION: The performances of CPAP devices are variable. The device that calibrated for the pressure loss in the circuitry under dynamic conditions and made appropriate pressure adjustments outperformed the other devices.