Blood gas measurements obtained during 35 episodes of acute, severe asthma in 19 children were analysed. Arterial carbon dioxide tension (PaCO2) was mean (SD) 5.7 (1.2) kPa and the arterial oxygen tension (PaO2) was 7.7 (1.1)kPa. Hypoxaemia was severe (PaO2 less than or equal to 7.9 kPa) on 19 occasions, was present alone (type I) on eight of these, and was associated with hypercapnia (type II) on 11. The PaO2 was similar in both the type I and type II subgroups, but PaCO2 was significantly higher in the type II and the alveolar-arterial oxygen tension difference was significantly higher in the type I subgroup. Classification of acute respiratory failure into these two types proved useful in understanding the pathophysiology of acute, severe asthma. Type I failure, conventionally regarded as a precursor of type II, itself caused severe, critical hypoxaemia.