The origin of neutrophils that are found in pleural effusions after pulmonary resection is unknown. We measured neutrophil counts in pleural effusion sequentially for 48 h in patients who had undergone partial resection or pneumonectomy. Additionally, we measured neutrophil counts in the pleural effusions separately from the visceral pleura and parietal pleura in rabbits. In humans, we found that the maximum neutrophil counts in pleural effusion occurred more often in the patients who underwent partial resection (11.7 +/- 6.0 x 10(4) cells/microL) than in the patients who underwent pneumonectomy (3.7 +/- 1.2 x 10(4) cells/microL). There was no difference between neutrophil counts in blood of pneumonectomy group and that of the partial resection group. In rabbits, neutrophil counts were 1.5 times greater in the pleural effusion derived from the lung than in that derived from the parietal pleura. We conclude that movement of neutrophils occurs primarily from the lung into the pleural space after lung resection in humans and rabbits.