The efficacy of continuous positive airway pressure (CPAP) administered with a face mask was compared to oxygen supplementation in 40 patients with acute cardiogenic pulmonary edema. Baseline arterial blood gas values and pH, heart rate, respiratory rate, and blood pressure were recorded after patients breathed 28 percent oxygen with an ambient airway pressure for ten minutes. Thereafter, inspired oxygen concentration (FIO2) was increased by 0.10 and airway pressure was increased by 10 cm H2O in random sequence. Cardiopulmonary function was reassessed after a five-minute stabilization period at each level of oxygen supplementation and airway pressure. An increase in FIO2 and the application of CPAP both produced significant elevation in arterial blood oxygen tension. Use of CPAP was consistently associated with a decrease in respiratory rate, blood pressure, and rate pressure product, which were not observed with a mere enhancement of oxygen therapy. The improvement in the measured cardiopulmonary variables associated with CPAP therapy cannot be achieved by reversing hypoxemia with supplemental oxygen only.