Objective　To study the diagnostic value of pleural fluid telomerase activity in distinguishing a malignant from a benign pleural effusion. Methods　Using a PCR-based assay, telomerase activity was examined in the pleural fluid cells obstained from 69 patients with pleural effusions (37 malignant, 32 benign). Results were compared with cytologic evaluation and carcinoembryonic antigen (CEA). Results　Of the 37 malignant specimens，26（70.27%）contained detectable telomerase activity. The positive rate in malignant pleural effusions was significantly higher than that in benign origin (2/32). Detection of telomerase in effusions was more sensitive than CEA level evaluation for the identification of pleural malignancy (telomerase assay: sensitivity=70.27%, specificity=93.75%; CEA: sensitivity=51.35%, specificity=96.87%). The consistant rate of telomerase with cytology was 54.05%. Conclusion　The detection of telomerase activity may be a useful adjunct to cytopathologic methods in the diagnosis of malignant pleural effusions. It may be useful in differential diagnosis of benign and malignant pleural effusions.