Two hundred and fifty pleural effusions were studied prospectively comparing the results obtained simultaneously by investigation of the pleural fluid, blind needle biopsy of the pleura and thoracoscopy. Thoracoscopy was necessary in 50% of the cases for the diagnosis or the exclusion of a neoplastic or tuberculous etiology. The combined cytological and histological results of the pleural effusions and the needle biopsies established the diagnosis of malignancy in 74%. Tuberculosis was proven histologically in 52% by needle biopsy, in 62% when the cases were not included where only inadequate tissue was obtained. If the bacteriological results of tb-cultures including those of the pleural effusions are added, 74% of the tuberculous pleurisies were diagnosed. Thoracoscopy had a significantly higher yield with 97% in malignant and 99% in tuberculous effusions. The value of thoracoscopy in pleural effusions of other or of idiopathic origins has to be seen in the light of these results showing the probability with which a tumour or a tuberculosis can be excluded, 9% of the cases remained indeterminate (idiopathic).