In order to evaluate the therapeutic effect of thoracostomy on the patients with pathological changes in pleural cavity which were caused by various etiological factors, a clinical study was carried out during a period of 5 and half years from May 1972 to September 1977 in the department of thoracic surgery, Hanyang University Hospital, and the following results were obtained. Of a total of 264 patients, 205 cases were male, and 59 female, exhibiting the ratio of male to female being 3.5 to 1. The pathological changes in pleural cavity could occur at any age from 4 months after birth to 76 years old, the peak incidence being in the third decade in either male or female. The incidence decreased in the second, fifth and fourth decade in order. The type of pathological changes observed and their frequencies of occurrences were 93 cases [35.2%] in pneumothorax, 62 cases [23.5%] in hemothorax, 48 cases [18.2%] in pyothorax, 46 cases [17. 4%] in hemopneumothorax, 13 cases [4.9%] in hydropneumothorax, and one case each in hydrothorax and chylothorax. The incidence of the primary diseases which predisposed the pathological changes in pleural cavity were, 119 cases [45-1%] in trauma, 64 cases [24.2%]in lung tuberculosis, 38 cases [14.4%] in pneumonia or empyema, 14 cases [5.3%] in lung emphysema and blebs, 13 cases [4.9%] in process after thoracotomy, 3 cases [1.1%] each in lung malignant tumor and lung paragonimiasis, one case in mechanical ventilator and 9 cases [3.4%] in unknown origin. The pathological changes in pleural cavity were located in the right side of the cavity in 124 cases, in the left side in 133 cases and in both sides in 7 cases, indicating that the difference between the incidence of the left and rightside occurences was insignificant. Of 93 cases of pneumothorax studied, 63 cases were found to have been tension pneumothorax and 30 cases non-tension pneumothorax, showing greater prevalence of tension type over non-tension type. Of 119 cases of trauma observed, 82 cases were accompanied with rib fractures and 37 cases were without any fracture [non-bone fracture]. Patients with the rib fractures were characterized by multiple rib fractures and multiple double fractures of ribs, accompanying with or without fracture of bones other than ribs, and patients with non-bone fracture were characterized by penetrating stab wound and blunt trauma. Of 264 cases who received thoracostomy, 207 cases [78.4%] demonstrated that their pathological changes in pleural cavity were removed and subsided by a simple measure of thoracostomy. In 43 cases [16.3%], various surgical measures including radical operation and thoracotomy were required for complete healing, since their pathological changes were not abolished by thoracostomy alone. The rest 14 cases [5.3%] were expired following thoracostomy.[KTCS 1977;2:205-213]