A review of the records of 227 patients with biopsy proved sarcoidosis disclosed roentgenographic evidence of pleural reactions (pleural effusion and/or pleural thickening) in 23 (10.1 per cent). Pleural effusions were noted in 15, and in all of these the disease process showed signs of progression to stage 2 or beyond. Transudative effusions were found in eight patients. Pleural biopsies in seven patients with effusion disclosed noncaseating granulomas consistent with sarcoidosis. An extensive evaluation for other underlying disease processes, in all cases was negative. The effusions resolved, but in two patients serial roentgenographic studies showed progression to chronic pleural thickening. Pleural thickening was observed initially in eight patients and was always associated with an advanced stage of sarcoidosis. Biopsies performed in five of these patients showed thickened fibrotic pleura interspersed with noncaseating granulomas. On review of lung biopsy specimens from 11 patients with no roentgenographic indication of pleural reactions, noncaseating granulomas in simultaneously obtained pleura were found in four. The pleura is affected in sarcoidosis more frequently than has been recognized. The occurrence of pleural involvement and reaction is a consequence of sarcoidosis and appears to be associated with the progression of the disease process.