The clinical features, arterial blood gases, and acid-base profile were examined in 229 consecutive episodes of acute asthma in 170 patients who required hospitalization. A simple respiratory alkalosis was the most common acid-base disturbance, occurring in 48 percent of the episodes. Metabolic acidosis, either alone or as part of a mixed disturbance, was noted in 28 percent. Of 60 episodes presenting with respiratory acidosis, 37 (62 percent) had a coexistent metabolic acidosis. Metabolic acidosis was more likely to occur in male subjects and in patients with evidence of more severe airflow obstruction. Patients with metabolic acidosis had an average anion gap of 15.8 mEq/L; these patients were more hypoxemic than those without metabolic acidosis and there was a significant inverse correlation between the anion gap and the degree of hypoxemia. We conclude that metabolic acidosis is a common finding in acute, severe asthma and suggest that the pathogenesis of lactic acidosis is multifactorial and includes contributions from lactate production by respiratory muscles, tissue hypoxia, and intracellular alkalosis.