A study of 10 infants in severe metabolic acidemia (pH below 7) led to the conclusion that the clinical signs-hyperventilation, coma or lethargy, peripheral vascular collapse, a significantly palpable liver, and abdominal distension-may all be directly related to the metabolic acidemia. In five of 10 infants, an initial erroneous diagnosis of congestive heart failure or pneumonia was made. Dramatic clinical improvement followed correction of the acidemia with rapid intravenous administration of sodium bicarbonate. This rapid administration of sodium bicarbonate was safe, provided hypocalcemia was recognized and treated in its early stages. In severe metabolic acidemia the measurement of blood bicarbonate alone does not reflect adequately the magnitude of the acid-base derangement and repeated measurements of hydrogen ion concentration, Pco(2) and bicarbonates are needed to evaluate and treat such infants correctly.