In 6 seated subjects with small unilateral pleural effusion without other radiographic abnormality, we measured regional lung function using xenon-133, comparing the function of the lung on the side with the effusion to the function on the contralateral side. When both lungs were equilibrated with Xe, count rates were lower at the base with the effusion, indicating either collapse or displacement of lung in these regions. Regional lung expansion was measured as fractions of regional total lung capacity: regional functional residual capacity/regional total lung capacity and regional residual volume/regional total lung capacity. The distribution of boluses inhaled slowly from residual volume was also measured, In all subjects the results were the same on the side with the effusion as on the other side. These findings were compatible with effective static pleural pressure being the same on both sides and suggest that the reduction in count rate at the base with effusion was due to displacement rather than compression of lung. We also measured regional washout half-times during dynamic breathing and found them prolonged in regions underlying effusion, indicating that dynamic ventilation of these areas was reduced. We postulated that the dynamic pressure swings applied to these regions were decreased by the effusions.