Arterial blood gas and pH measurements in 82 patients with acute pulmonary edema of cardiogenic origin entering the emergency department varied widely and were unpredictable using clinical examination. The mean arterial oxygen tension (PaO2) measured in 71 patients breathing room air was 59 mm Hg. Fourteen of the 82 patients were acidemic; 35, alkalemic and 33 had a pH in the normal range. The acidemic group had markedly lower PaO2, all under 60 mm Hg. Oxygen and furosemide were used in all cases and effectively corrected the hypoxia and reduced pulmonary congestion. Other drugs used included aminophylline (14 patients), morphine sulfate (9 patients) and digoxin (3 patients). Five of the nine patients who received morphine were hypercarbic initially but the CO2 retention did not worsen. No patient died during the initial 48 hours. This study reiterates the importance of directing therapy at ventilatory and cardiac abnormalities and points out the value of arterial blood gas monitoring to assess the initial status, monitor the patient's course, and to select drug therapy.