We have studied the serial changes in arterial blood gases in fourteen patients with acute severe asthma, all of whom received a standard therapeutic regime and had similar measurements made at identical time intervals under standard conditions. Hypoxaemia on admission was a constant finding, and the arterial oxygen tension often took a week or longer to return to a normal level. Treatment with 60% inspired oxygen provided a safe means of relieving hypoxaemia, provided that blood gases were measured before and during oxygen therapy. Most patients had a normal arterial carbon dioxide tension, which indicated the severity of their illness. Acid-base disturbances, when present, were mild and needed no specific treatment. Age, duration of the acute attack, and severity of airways obstruction were all unrelated to the changes in blood gas tension, and pulse rate was found to be a poor predictor of hypoxaemia in elderly asthmatics. Serial measurements of the arterial blood gases should be made in all patients with acute severe asthma.