This study was undertaker to define the usefuness of preoperative CA-125 assay as a diagnostic bmor marker in differentiating malignancy from benign ovarian mass. Senun CA-125 were imneasured by Microparticle Enzyme Immunoassay(MEIA) in 94 patients with ovarian mass. The results were of follows ; 1. The mean value of preopentive senun CA-125 was 18.40u/ml in benign ovarian mass and 225.99u/ml in malignant ovarian mass (P<0.001). 2. The positive rete of Ca-125 in benign ovarian mass was 10%, compared 80% in malignant ovarian mass. 3. In analysis of histolovgic type, posisitive rate of serum CA-125 in malignant serous tumor was 82%, cornpared 50% in malignant mucinoins tumor. 2. No statistically significant correlation was observed between CA-125 value and patient's age. 5. The sensitivitiy, specifieity, positive predictive value & negative predictive value were 80%, 90%, 60% & 96%, respectively in cut off value, 35u/ml, And increasing cut off value 65u/ml, sensitivity, specificity, positive predictive value & negative predictive value were 40%, 96%, 67%, 90%, resqxetively. These data suggest the preperative serum CA-125 level correlate with maignant stattis in ovarian mass. And cut off value 35u/ml was better than 65u/ml in screening for ovarian cancer.