[en] Objective: To evaluate the differential diagnostic significance of determination of CK19 mRNA contents in the effusion fluid in malignant and benign hydrothorax. Methods: Pleural effusion fluid CK19 mRNA contents was examined with RT-PCR in 43 cases of malignant hydrothorax nd 57 cases of benign hydrothorax. Results: In the patients with malignant hydrothorax, positive rate of CK19 mRNA in pleural effusion fluid was significantly higher than that patients with benign hydrothorax (P < 0.005). The sensitivity of CK19 mRNA detection was 90.7%, the specificity was 70. 2% and the accuracy of CK19 mRNA detection was 70.2%. Conclusion: Detection of CK19 mRNA in pleural effusion fluid could be a promising method for differential diagnosis of benign from malignant cases. (authors)