The ventilatory response to hypoxia and the ventilatory and mouth occlusion pressure response to hypercapnia was measured in 13 subjects who had previously developed respiratory failure or respiratory arrest during an acute asthma attack. In 11 of 12 subjects tested there was a normal response to hypercapnia. Six of the 13 subjects had an impaired response to hypoxia. Impaired hypoxic responsiveness may contribute to the early onset of hypercapnic respiratory failure during acute severe asthma.