Abstract: The effecis of pleural effusion in sixty patients and the effects of fibrothorax in 70 patients were described. The patients were submitted to spirometry and arterial blood gas analysis. The patients with pleural effusion were divided into three groups according to varying magnitude of pleural effusion. The effects of large-size pleural effusion on lung function and arterial oxygen tension were surprisingly large: VC was only 54.2% of predicted value; FVC was 56.9; FEV₁48.5%; TLC 62.9%; RV 86.4%; RV/TLG 118.6%; FEF50 42.3%; FEF25 36.5%; PaO2 78.1%; and SaO2 93.3%. On the other hand, the effects of small-size pleural effusion on pulmonary function are small: PaO2 of the patients with small-size pleural effusion was not significantly reduced as compared with predicted value. In patients with pteural effusion and fibrothorax pattern of pulmonary function is similar.