Although continuous positive airway pressure (CPAP) therapy using a face mask is known to improve oxygenation, the intrapulmonary shunt reduction remains unsettled. Our study was designed to explore this issue. From 1985 to 1987 80 patients with acute cardiogenic pulmonary edema were randomly chosen to receive either serial CPAP therapy or high-flow face mask oxygen therapy without CPAP (control) for the purpose of evaluating the efficacy of CPAP therapy. After screening for exclusion, only 55 patients were included in the first 3-hour investigation period. PaO2 in the CPAP group showed a significant increase by the end of the initial study; whereas intrapulmonary shunt and alveolar-arterial oxygen tension gradient AaDO2 revealed simultaneously a significant reduction. Conversely, neither PaO2 nor intrapulmonary shunt (or AaDO2) in the control group demonstrated any significant beneficial changes. As for cardiovascular function, only the CPAP therapy achieved a significant reduction in rate pressure product in contrast to the control (face mask) therapy alone. In terms of therapeutic failure, 10 patients in the control group failed. However, in the CPAP group only 5 patients had failed at the end of the first 3-hour study period. Overall, the cumulative therapeutic failure rate was 28% in the CPAP group and 60% in the control group during a 6-hour observation study. However, there was no significant difference between the two groups in 24-hour hospital mortality.(ABSTRACT TRUNCATED AT 250 WORDS)