To compare the effects of continuous positive airway pressure (CPAP) with those of ambient end-expiratory pressure (T-tube) on lung mechanics and blood gas exchange, transpulmonary pressure (Ptp), tidal volume (VT), respiratory frequency, and arterial oxygen and carbon dioxide tensions were measured in 16 spontaneously breathing patients recovering from acute respiratory failure. These variables were measured during breathing through a T-tube; with 18, 12, and 6 cm H2O CPAP; and again during breathing through a T-tube. During all levels of CPAP, mean effective lung compliance (Ceff) was higher and mean total pulmonary power during inspiration lower than during breathing through a T-tube before CPAP (P< 0.05). The data obtained at the level of CPAP producing maximum Ceff(optimum CPAP) were grouped and compared with values obtained during breathing through a T-tube. Mean total pulmonary power of inspiratory muscles during breathing through a T-tube before CPAP (0.7 ± 0.14 kg.m.min-1) decreased during optimum CPAP (0.44 ± 0.07 kg.m.min-1) and increased during breathing through a T-tube after CPAP (0.63 ± 0.12 kg.m.min-1). Mean VTwas higher (557 ± 63 mlvs.474 ± 47 ml) and frequency lower (17.5 ± 1.6 breaths/minvs.22.5 ± 2.5 breaths/min) during optimum CPAP than during breathing through a T-tube before CPAP, and inspiratory time was significantly longer. Mean minute ventilation was also lower during optimum CPAP (8.7 ± 0.6 1/min) than during breathing through a T-tube (9.6 ± 0.8 1/min); Paco2did not change significantly. Mean alveolar-to-arterial oxygen pressure difference decreased significantly during optimum CPAP. The authors conclude that CPAP, when adjusted to the appropriate levels, improves lung mechanics in patients recovering from acute respiratory failure. Continuous positive airway pressure reduces total pulmonary power during inspiration and at the same time improves oxygen and carbon dioxide exchange. In these respects, it is preferable to breathing through a T-tube without CPAP.