1. Ventilatory response to CO2 was measured regularly by a rebreathing technique in nineteen patients with severe asthma from the day of presentation to the time of clinical recovery.
2. Ventilatory response to CO2 increased during recovery in sixteen patients and the increased ventilatory response correlated well with increase of FEV1. Among these sixteen patients only one showed elevation of arterial CO2 tension at the time of presentation.
3. Ventilatory response to CO2 failed to increase during recovery in three patients despite increases in FEV1. All three patients showed elevation of arterial CO2 tension at the time of presentation.
4. In five patients (including three of the four with initial hypercapnia) ventilatory response to CO2 after recovery remained below the previously reported lower limit for normal subjects. The limits of normality were explored by examining ventilatory response to CO2 in seventeen outstanding athletic performers. Values for ventilatory response to CO2 both above and below the previously defined ‘normal range’ were found. The normal ventilatory response to CO2 covers a 14-fold range from 0.57 to 8.17 1 min−1 mmHg−1Pco2.