22 asthmatic and 31 chronic-bronchitic patients with comparable airway obstruction had hypoxæmia of similar extent but the bronchitics had significantly greater arterial carbon-dioxide tensions than the asthmatics. Forced expiratory volume in 1 second (F.E.V. 1) increased in 4 asthmatics after isoprenaline, but there was no improvement in hypoxæmia. Reasons are given for regarding acute hypercapnia in the asthmatic as an important sign of impending deterioration, and the need for blood-gas-tension measurements in the management of the severe asthmatic is stressed.