We reviewed our experience with fiberoptic bronchoscopy (FOB) in patients with pleural effusion of unknown origin. Seventy patients underwent FOB for the investigation of pleural effusion between 1978 and 1983. Those with a second reason for FOB, a mass on chest roentgenogram, or lobar atelectasis were excluded. Forty five patients remained: 28 patients with unexplained pleural effusion after pleural fluid analysis and pleural biopsy (UPE), and 17 patients with malignant pleural fluid cytology and/or pleural biopsy but no known primary tumor (MPE). In the UPE group, only one FOB demonstrated malignancy, despite a final diagnosis of tumor in seven. No other specific diagnoses were made by FOB in this group. In the MPE group, FOB demonstrated bronchogenic carcinoma in two; ultimately, five patients were found to have a bronchogenic neoplasm. Although pleural effusion of unknown origin is frequently caused by bronchogenic carcinoma, FOB in the absence of other indications for this procedure is rarely diagnostic and should not be routinely employed.