From 200 observations, the authors study the results of laboratory examinations and paraclinical testings of the pleura. Neoplastic etiologies represent a 1/3 of all pleurisies. In 20% of cases a definite etiological diagnosis could not be obtained in spite of exploring thoracotomies. The limit of 30 g/l of proteins for the differentiation of mechanical transsudates and inflammatory exsudates was inaccurate. Cytodiagnosis was efficient in 75% of mesotheliomas but the percentage fell to 53 in secondary cancers. For needle biopsy results were respectively of 66% and 57%. Thoracotomy (24 cases) enabled a diagnosis in half the cases where it was performed. Thorascopy seems to bring similar results.