Acute, often severe, respiratory acidosis which cleared within several hours was noted in ten patients with classic cardiogenic pulmonary edema. Mental obtundation was present in five. Only three of the ten patients had received narcotics. Treatment was directed at the pulmonary edema and usually included intermittent positive pressure breathing. Coexisting metabolic acidosis was mild; two patients had associated metabolic alkalosis. On presentation, the mean Pco2 was 64.9 mm Hg and mean pH 7.20; after several hours of treatment, pH was normal and Pco2 minimally reduced. In view of these findings in pulmonary edema, emphasis should be placed on analysis of the arterial blood gases, a free airway, and supported ventilation.