Diagnostic thoracoscopy is indicated in every patient where the usual investigations (including biochemistry, cytology, bacteriology, occasionally needle biopsy of the pleura) do not achieve a precise diagnosis. The percentage of so-called "idiopathic" effusions, amounting to approximately 20% in many published series, can be reduced to 4% after thoracoscopy. The sensibility of thoracoscopic biopsy reaches 93-97% of malignant or tuberculous pleural effusions. The procedure requires a short hospitalization of about 36 hr, and complications are rare. Therapeutic thoracoscopy is frequently performed in chronic, malignant, recurrent effusions in order to achieve a pleurodesis by means of a talc poudrage under visual control. The efficacy of the poudrage in the published randomized studies is better than tetracycline. About 90% of patients are cured, the effusion being totally suppressed. Side effects are rare if the quantity of talc does not exceed 10 ml.