Parietal pleural biopsy, together with a bacteriological and chemical analysis of the pleural fluid, is often the most useful diagnostic technique in pleurisy. This is a simple technique, with moderate aspiration across a transparent filter trap, as used in blood transfusions. The procedure is done under direct vision and has the following advantages : it improves the specificity of the pleural material aspirated. It is without danger, can be repeated a number of times and thus increases the diagnostic value of the test. It avoids pleural superinfection and instrumental pneumothorax. It improves safety for the patient and comfort for the operator. It improves on the usual method for needle-biopsy with or without a syringe.