BACKGROUND: Pleural effusion is a common disease in clinical practice but its effect on pulmonary function and altered pulmonary mechanics after removal of effusion are not still largely understood. Previous studies have shown that there is little or a relatively small improvement in pulmonary function and arterial blood oxygenation after therapeutic thoracentesis. The present study was designed to assess the effect on pulmonary function of pleural effusion and to test whether there was a significant improvement in pulmonary function and arterial oxygenation after thoracentesis and to observe long term effect after thoracentesis. METHOD: We examined flow-volume curve, body box and arterial blood gas analysis according to severity of effusion, present symptom, and symptom duration. Then, we measured changes of pulmonary function after thoracentesis and observed longterm effect after thoracentesis. RESULT: 1) Pleural effusion cause restrictive pulmonary insufficiency. Not only functional impairment of small airway but also large airway is provoked. 2) MMFR, FEV1, Raw, PO2 are earlier improved than FVC and TLC after thoracentesis and patients without complication have mild restrictive pulmonary insufficiency after longterm observation 3) FVC, FEV1, & TLC are similarly restricted as severity of pleural effusion and PO2 is relatively decreased. 4) Cases with symptom duration 1 week or less and cases with dyspnea have more severe pulmonary insufficiency than others. 5) The flow volume curves show a relatively greater improvement in flow rates at large lung volumes than small airway. 6) Significant relationship is shown between first thoracentesis amount and changes of FEV1, FVC, TLC. CONCUSION: Pleural effusion cause restrictive pulmonary insufficiency and not only functional impairment of small airway impairment but also large airway is provoked. Then, Pulmonary function is progressively improved after thoracentesis and remained mild restrictive pulmonary insufficiency after recovery.