Nowadays, the diagnosis of pleural effusion is greatly facilitated by thoracoscopy. Since most pleural effusions are due to neoplasias (notably in patients older than 40 years) and since the prognostic value of an early diagnosis is obvious in case of pleural carcinoma (notably mesothelioma), there is no justification in allowing the effusion to become chronic. If the diagnosis is confirmed by needle biopsy, thoracoscopy can rapidly be performed by skilled operators: in 95 p. 100 of the cases it provides the diagnosis. Five per cent of pleural effusions remain of unknown origin. In such cases the patients must be closely followed up for 12 to 18 months to make sure that no cancer has been missed and another thoracoscopy must be performed at the slightest clinical change. Test treatments (antibuberculous drugs, corticosteroids) are now obsolete.