Background and objective Carbonic anhydrase IX (CAIX) is widely expressed in a variety of malig-nant tumors, including-lung cancer. Our previous study has shown that the serum level of soluble form of carbonic anhydrase IX (s-CAIX) was signiifcantly higher in patients with lung cancer than that in the healthy group. hTe aim of this study is to detect the s-CAIX level in the pleural effusion and its sediment, and to evaluate the signiifcance of CAIX detection in the diag-nosis of lung cancer.Methods hTe s-CAIX level in pleural effusion of 29 lung cancer patients and 27 patients with tuberculosis was detected by ELISA. hTe expression of CAIX in the pleural effusion sediment of 21 lung cancer patients with malignant pleural effusion and 6 patients with benign pleural effusion was examined by immunohistochemistry. With pathological diag-nosis as the gold standard, receiver operating characteristic (ROC) curve of pleural effusion s-CAIX was established for the diagnosis of lung cancer with malignant pleural effusion.Results hTe s-CAIX level in the malignant pleural effusion was signiif-cantly higher than that in the tuberculosis group (P<0.05). hTe AUC of pleural effusion s-CAIX level was 0.761. At a threshold level of 109.135 pg/mL, sensitivity and speciifcity were 92.3% and 58.3%, respectively. hTe CAIX expression in all samples of the benign pleural effusion sediment was negative. hTe positive rate of CAIX expression in malignant pleural effusion sediment was 66.67%.Conclusion Detection of CAIX in the pleural effusion and its sediment exhibits high sensitivity and speciifcity, and is helpful in diagnosis of lung cancer with malignant pleural effusion.