123 patients with chronic pleurisy were investigated by thoracoscopy. The endoscopic biopsy is positive in 16 out of 17 cases of mesothelioma (96%), in 57 out of 72 cases of pleural metastasis (79%), and in 6 cases out of 7 pleural tuberculosis. Begnin pleural asbestosis was confirmed in 5. Endoscopic biopsy is thus more efficient than results obtained with pleural needle biopsy and exudative cytology. Thoracoscopy can be performed without any serious complications, this is not always true with open thoracotomy.