When performing thoracoscopy in patients with pleural effusion of unknown origin, we used two bronchoscopes simultaneously, one for observation and one for biopsy. A total of 50 patients with pleural effusion of unknown origin were studied. In all of those studies, pleural effusion was exudative, lymphocyte-dominant, had a low level of adenosine deaminase, no malignant cells, and no tuberculosis or other bacteria in pleural effusion smears. Fourteen were out-patients. A catheter was inserted into the pleural space under local anesthesia, and 300 ml to 500 ml of pure oxygen was injected to create a pneumothorax. Two flexible fiberoptic bronchoscopes were used simultaneously, one for observation and one for biopsy. Approximately 1 hour after the examination, the out patients were able to return home. Lesions in the pleural cavity were found in 42 of these 50 patients, and histological diagnosis was possible in 46. This is a simple procedure with no major side effects. The equipment required is familiar to pulmonary physicians, and the diagnostic yield is high.