The effect of body position (right and left lateral decubitus positions) on arterial oxygen tension (PaO2) and the relationship between this postural effect on gas exchange and pulmonary function were evaluated in 21 patients who had unilateral pleural effusions without roentgenographic and bronchoscopic evidence of bronchopulmonary disorders. Our results indicated that a positional influence on gas exchange existed in these patients. We failed to find a consistent relationship between the size of effusion estimated by chest roentgenogram and alterations in PaO2 during different positions. Postural change did affect gas exchange in the patients with unilateral pleural effusions and this postural effect on gas exchange was highly correlated with their FEV1 and FVC. This may be of clinical significance in managing such patients.