ObjectiveTo investigate the expression level of exosome microRNA-21 (miRNA-21) in the bile and its clinical diagnostic value for the patients with cholangiocarcinoma. MethodsIn this study, 45 cases of cholangiocarcinoma admitted to Dongfeng General Hospital from August 2019 to December 2021 and met the inclusion criteria were selected and 35 patients with benign diseases of bile duct (choledocholithiasis or benign stricture of bile duct) during the same period were selected as control. The exosome in the bile was extracted by hypervelocity centrifugation method and identified. The exosome miRNA was extracted from the bile using a kit, then the expression level of miRNA-21 was detected by real-time fluorescent quantitative PCR. The diagnostic value of exosome miRNA-21 in the bile for cholangiocarcinoma was analyzed by receiver operating characteristic curve (ROC). ResultsThe isolated exosome in the bile conformed to the characteristics of recognized exosome and the concentration was higher. The average expression level of exosome miRNA-21 in the bile of the patients with cholangiocarcinoma was statistically higher than that in the patients with benign diseases of bile duct (59.45 verses 25.41, t=3.445, P<0.001). The area under the ROC curve was 0.715 [95%CI (0.602, 0.827), P=0.001]. The sensitivity and specificity of miRNA-21 in the diagnosis of cholangiocarcinoma were 75.6% and 62.9%, respectively. ConclusionFrom the results of this study, exosome miRNA-21 expression in bile is higher and it may be a potential early diagnostic marker for patients with cholangiocarcinoma.