CA 125 is an antigenic determinant on a high molecular weight glycoprotein. A monoclonal antibody has been produced which recognizes this, and allows us to measure the expression of CA 125 in serum. Tissue distribution of the CA 125 determinant is most commonly seen in serous tumors of the ovary, with highest levels in borderline and frankly malignant serous cystadenocarcinomas. Occasionally cancers of the breast, gastrointestinal tract, and kidney will show elevated levels of CA 125. Normal tissues which show varying levels of CA 125 include decidual tissue and structures derived from celomic epithelium. CA 125 is clearly tumor associated, but not tumor specific. Quantitative correlation of CA 125 levels with tumor volume has not been demonstrated. This observation limits the clinical usefulness of CA 125 as a screening tool, particularly in premenopausal patients who do not have a diagnosis of ovarian cancer. An undetectable level of CA 125 antigen does not rule out the presence of an early ovarian cancer. When CA 125 is used to monitor disease state in patients with known ovarian cancers (whose tumors do express CA 125), changes in levels of CA 125 do correlate with gross changes in tumor volume. Good prognostic significance is attributed to a rapid decline in CA 125 levels following induction chemotherapy in patients with advanced ovarian cancer. However, an undetectable serum level of CA 125 does not predict clinical cure for a patient with ovarian cancer. Further clinical studies continue in the use of CA 125 as a screening tool and as a means to monitor treatment responses of known ovarian cancer.