Up to 25% of malignant pleural effusions can remain undiagnosed following history, physical examination, thoracentesis, and percutaneous closed pleural biopsy. The next diagnostic procedure is often rigid thoracoscopy, an invasive procedure requiring an operating suite and usually a postprocedure chest tube. We performed flexible fiberoptic pleuroscopy using a fiberoptic bronchoscope in conjunction with a closed pleural biopsy on 12 patients with exudative pleural effusions that remained undiagnosed despite extensive clinical evaluation. A sterile 4.8-mm outside diameter flexible fiberoptic bronchoscope was placed into the pleural space during the course of a routine closed pleural biopsy. Pneumothorax was induced to allow visualization. Brush or forceps biopsy specimens of suspicious parietal pleural lesions were taken. Eight pleural spaces appeared smooth while four were diffusely studded on the parietal surface. Of these four, three were proven to have diffuse pleural adenocarcinoma using this procedure; the fourth proved ultimately to have pleural mesothelioma. On long-term follow-up (mean = 17.7 +/- 11.4 months), no false-negative studies or unexpected morbidity was noted. Flexible fiberoptic pleuroscopy may provide a diagnosis in exudative pleural effusions when other less invasive procedures fail to do so and is well tolerated with minimal discomfort and risk.