The study was done on 30 patients in Kasr El-Aini hospital, chest department, who had pleural effusion of different aetiologies. They were further subdivided into two groups; exudates and transudates, levels of both serum and pleural fluid CA-125 were evaluated; it was found that CA-125 (serum and pleural)was high in both exudative effusion and transudative effusion, mainly malignancy, and that there is positive correlation between serum and pleural fluid CA-125.Also, it was found that its sensitivity is high but specificity is low as high levels of CA 125 were detected in cirrhosis, peritonitis, pancreatitis, benign ovarian cysts, and malignant tumors. The origin of CA 125 is defined as not only ovarian cells, but also peritoneal, pleural, pericardial and endometrial cells and benign ovarian cysts.