Abstract: Objective: To determine the diagnostic value of pleural fluid telomerase activity in distinguishing a malignant from a nonmalignant pleural effusion.Methods: Using a PCR(polymerase chain reaction)-based TRAP(telomeric repeat amplification protocol) assay combined with ELISA(enzyme-linked immunosorbent assay),we examined telomerase activity in the pleural fluid cells obtained from 43 patients with malignant neoplasms and 24 patients with benign lesions.The result was compared with that by cytological diagnosis and determination of carcinoembryonic antigen(CEA).Results: The presence of telomerase activity was observed in 32 of 43 samples of malignant pleural effusions,and in 3 of 24 samples of nonmaligant pleural effusions.The difference was significant(P<0.005).The detection of telomerase activity had a sensitivity of 74.42% to the diagnosis of malignant pleural effusions and a specificity of 87.5%,respectively.The overall positive rate in 43 malignant pleural effusion cell samples by cytological examination and by the determination of CEA was 51.16% and 46.51%,respectively.The positive rate detected by cytological examination and CEA was significantly lower than that by telomerase activity(P<0.01).Conclusions: Our results indicate that the detection of telomerase activity is of great value in the diagnosis of malignant pleural effusions.The diagnosis will be more reliable of combined with cytological examination and CEA determination.