In 249 patients with ventilatory insufficiency the PaCO2 predicted using Burrows’ formula 11.5/FEV1•36.7 mmHg was always lower than the one measured in a blood gas apparatus. This occurred independently of the degree of oxygenation of the arterial blood and it did not make any difference if the FEV1 used was the one obtained before or after bronchodilators. This lack of correlation between predicted and measured PaCO2 is explained by individual variati