Respiratory acidosis of severe acute asthma is a severity factor. In this paper the treatment of associated metabolic acidosis is discussed. Among 34 consecutive episodes of severe acute asthma with acidosis (pH < 7.35) treated with continuous adrenaline perfusion, theophylline and hydrocortisone hemisuccinate, respiratory acidosis was observed in 12, metabolic acidosis in 2 and mixed respiratory and metabolic acidosis in 20. The association of hypercapnic acidosis with hypochloraemic acidosis reflected a time of installation longer than when respiratory acidosis only was present (p < 0.05). Among the 22 patients who had metabolic acidosis on admission, 14 were treated with 168 +/- 82 mmol of sodium bicarbonate, the remaining 8 patients being untreated and acting as controls. The rapidity with which pH was corrected was the same in the treated and untreated groups (9.1 +/- 5.5 hours vs 6.7 +/- 3.7 hours), whereas dyspnoea (respiratory rate < 18/min) was more rapidly corrected in the treated group that in controls (11.6 +/- 5.7 hours vs 5.9 +/- 5.9 hours; p < 0.05). It is concluded that in more than 50% of the cases respiratory acidosis of severe acute asthma is associated with a metabolic acidosis. Correcting this metabolic acidosis with sodium bicarbonate results in improvement of respiration, perhaps by facilitating the action of bronchodilator catecholamines.