This study was undertaken to determine whether the measurement of CA125 could effect an early diagnosis and a method for monitoring the course of gynecologic tumors. CA 125 in serum of 195 patients, including 15 apparently healthy women; 39 benign gynecologic tumors; 2 borderline ovarian tumors; 139 malignancies were measured by CENTOCOR cancer antigen 125 kits. Diagnosis of all patients was confirmed by pathology. None of the healthy women; 10% of benign tumors; 78% of epithelial ovarian cancers; 31% of endometrial adenocarcinomas and one out of five adenocarcinoma of uterine cervix had CA 125 level over 65 U/ml. In addition, 23 cases of ovarian cancer were monitored serially up to 9 months. In more than 80% of these patients, CA 125 levels were correlated with the regression or progression of the disease. The significance of this assay for early diagnosis and monitoring the course of ovarian cancer is discussed. It is considered that CA 125 is a promising and useful antigenic marker for monitoring the course of ovarian cancers.