Acid-base abnormalities in adults with acute asthma are highly variable. The major abnormalities have been attributed to changes in PaCO2 secondary to altered alveolar ventilation. Primary metabolic alterations have been thought to be rare in adults. The frequency of metabolic acidosis in acute asthma was reevaluated by way of a review of the medical records of 164 hospitalizations involving 109 adults. The hydrogen ion concentration varied widely and in 27 of 164 episodes (16.5%) the arterial pH was lower than expected for the measured PaCO2. We conclude that the presenting acid-base status of adults with acute asthma is highly variable and metabolic acidosis is not an infrequent finding.