Abstract: ［Abstract］ Objective To evaluate the clinical value of medical thoracoscopy in the diagnosis of exudative pleural effusions with unknown etiology．Methods We analyzed retrospectively the clinical data of 180 patients with undiagnosed pleural effusions who underwent medical thoracoscopy and appraised clinical value regarding validity，effectivity and applicability of thoracoscopy．Result In 180 cases of pleural effusion，there were 178 cases of patients with a clear diagnosis，including 87 cases of tuberculous pleurisy，28 cases of malignant tumor，malignant breast bildes mesothelioma in 2，lung cancer in 24，bile duct carcinoma in 1，cercival cancer in 1），pneumonia 58 case fo pyothorax，5 caseso of pyothorax，and 2 cases withou clear diagnosis.No check failure and terminate，the 95%CI was 5 cases of 97.78%-99.98%．Conclusions　Medical thoracoscopy is a safe and effective method in the diagnosis of exudative pleural effusions with unknown aetiology .The early application of this method is very helpful for the management of the pleural diseases in suitable patients.