To assess the value of thoracoscopy in undiagnosed pleural effusion.By thoracoscopy, 114 cases of unknown origin pleural effusion were examined. Abnormal pleurae were biopsied in sight of thoracoscopy and compared with blind needle biopsy for pleurae.Positive rate was 91.2% (104/114) by thoracoscopy and 28.1% (32/114) by blind needle biopsy for pleurae. Significant difference between the groups was found (P < 0.005). In sight of thoracoscopy the morphology of pleural mesothelioma, metastatic carcinoma and tuberculous pleurisy was different. The rate of malignancy of sanguineous pleural effusion was 77.1% and yellow pleural effusion was 53.0%. PPD test showed that intense positive reaction was 42% in the benign cases, but negative response was 48% in the malignant cases. No serious complication occured in all cases.Thoracoscopy is an safe and effective measure in diagnosing patients with undiagnosed pleural effusion.