Author made a clinical study of 248 cases of pleural effusion patients who were diagnosed and treated at departments of chest surgery and internal medicine, Pusan National University Hospital, during the period from Jan. 1983 to Dec. 1985. The age distribution ranged from 1 to 76 years old and the ratio of male to female was 1.38:1. The cardinal symptoms were chest pain[69.4%], dyspnea[66.1%], cough[57.7%], fever[37.1%], sputum[26.2%], general malais[13.7%] and cyanosis[1.6%] in this order. The causes of pleural effusion were pulmonary tuberculosis[42.4%], pneumonia[23.0%], malignancy[16.5%], congestive heart failure[9.3%], liver cirrhosis[2.8%] and nephrosis[2.0%] in this order. The protein in the pleural effusions was 1.61*0.90[mean*SD] gm% in transudate and 5.05*1.10[Mean*SD] gm% in exudate. In 34 cases[89.5%]out of 38 transudates, the protein was under 3 gm% and in 201 cases [95.7%] out of 210 exudates, the protein was over 3 gm%. The protein ratio of pleural effusion to serum was 0.2650.11[Mean LSD] in transudates and 0.73*0.12[Mean LSD] in exudate. The ratio under 0.5 was in 36 cases[94.8%] out of 38 transudates and over 0.5 was in 206 cases[98.1%] out of 210 exudates. The LDH in the pleural effusion was 114.7550.3[mean*SD] units / ml in transudate and 627.05325.9[mean*SD] units / ml in exudate. The LDH less than 200 units / ml was in 36 cases[94.6%] out of 38 transudates and more than 200 units / ml was in 199 cases[94.7%] out of 210 exudates. The LDH ratio of pleural effusion to serum was 0.34k 0.11[mean*SD] in transudate and 1.15*1.12[mean*SD] in exudate. The LDH ratio of pleural effusion to serum was less than 0.6 in 36 cases[94.8%]out of 38 transudates and more than 0.6 in 200 cases[95.2%] out of 210 exudates. Etiologic organisms were confirmed in 78 cases[48.1%] among the requested 162 cases. In the 78 cases of etiologic organisms, staphylococcus was 33 cases[20.3%], streptococcus 24 cases[14.8%], Klebsiella pneumonia 7 cases[4.3%], pseudomonas 6 cases[3.7%], E. coli[3.1%], enterobacter 3 cases[1.9%]. 43 patient of pleural effusion from malignancy were undergone three or more thoracenteses. In 13 cases[31.7%], three specimen were negative and in 7 cases[17.1%], three specimens were positive for malignancy. In the remaining of 21 cases[51.2%], malignant cells were found in one or more of the specimens but not in all. Methods of treatment of pleural effusion by closed thoracotomy was 188 cases[75.8%], thoracentesis 27 cases[10.9%], decortication 16 cases[6.5%], thoracoplasty 6 cases[2.4%] and decortication with thoracoplasty 3 cases[1.2%].