Previous studies have established spirometric criteria for arterial blood gas analysis during acute asthmatic attacks. However, only general guidelines have been available regarding the need for blood gas analysis during an acute exacerbation of chronic obstructive pulmonary disease (COPD). We conducted a study to determine the relationship between arterial blood gases and spirometry in 70 emergency department patients during acute exacerbations of COPD. Arterial blood gas analysis and spirometry were performed on arrival at the emergency department. All of the patients with a pCO2 of more than 45 mm Hg had an FEV1 of less than 35% of the predicted normal. We found patients with a pO2 of less than 60 mm Hg who had an FEV1 as high as 54% of the predicted normal. There was no correlation between the FEV1 and pO2. Because spirometry was not reliable for identifying patients with significant hypoxemia, we conclude that arterial blood gas analysis is indicated for patients presenting to the ED with acute exacerbations of COPD. Spirometric criteria that have been used to eliminate the need for arterial blood gases in asthmatic patients cannot be applied safely to patients with COPD.