Background: Continuous positive airway pressure(CPAP) during spontaneous breathing has contributed greatly to the management of the neonatal respiratory distress syndrome and adult respiratory failure associated with acute lung disease. CPAP systems may be grouped into two general categories, demand flow CPAP system and continuous flow CPAP system. The purpose of this study was to evalute the respiratory effects of continuous flow CPAP system compared with demand flow CPAP system. Methods: Cardiorespiratory values were measured in twelve respiratory failure patients on demand flow CPAP system and continuous flow CPAP system made by authors. CPAP level and FIO were maintained at the same level for both modalities. Results: Changing from demand flow CPAP to continuous flow CPAP was associated with significant decrease in tidal volume(demand flow CPAP 450±153.0 ml, continuous flow CPAP 338±73.8 ml), airway pressure fluctuation(demand flow CPAP 6.4±1.2 cmH₂O, continuous flow CPAP 2.4±0.7 cmH₂O) and improvement in arterial oxygen partial pressure(demand flow CPAP 90.0±20.9 mmHg, continuous flow CPAP 105.9±24.6 mmHg). There were no significant changes in other cardiorespiratory values, such as arterial blood pressure, heart rate and respiratory rate. Conclusions: These results suggest that continuous flow CPAP system may be a beneficial modality in the management of respiratory failure patients compared to demand flow CPAP system. (Korean J Anesthesiol 1996; 31: 64-69)