Abstract: Pleural fluid and serum levels of CA-50 and diamine oxidase (DAO)were determined in 27patients with malignant effusion and in 18 patients with benign effusion. Pleural fluid and serum levels of CA-50 and DAO in the patients with malignant effusion were significantly higher than in those with benign effusion. If the CA-50 cut-off levels in pleural fluid and serum were all set as20 u/ml, the sensitivity rates were 77. 8% and 59.2%,the specificity rates were 94. 4% in both.when the DAO cut-off in pleural fluid was set as 250 u/ml, the sensitivity rate was 70. 4%, thespecificity rate was 88. 9%, There was no correlation between CA-50 and DAO in pleural effusion. The assay of CA-50 combined with DAO in pleural fluid increased the positive rate in comparision with CA-50 assay alone or DAO assay alone.