We studied 117 patients with chronic obstructive pulmonary disease (COPD) to evaluate (1) the frequency and magnitude of postural changes in resting arterial oxygenation and (2) the relationship of these changes to other measures of pulmonary function and exercise arterial blood gases. Compared to the supine measurement, room air PaO2 measured while standing increased more than 3 mm Hg in 28 patients (group 1), did not change (+/- 3 mm Hg) in 57 patients (group 2), and decreased more than 3 mm Hg in 32 patients (group 3) (range = 31 mm Hg increase to 20 mm Hg decrease). Patients in group 1 had significantly less severe disease than patients in the other two groups. There were no significant pulmonary function differences between groups 2 and 3. Supine PaO2 was similar for all groups, suggesting that standing PaO2 accounted for the postural change in PaO2. Because of unpredictable postural changes in PaO2 in patients with COPD, we believe that body position should be noted for arterial blood gas measurements and should be kept constant for valid comparison of serial measurements. These findings may also be important for other diffuse lung diseases.