The tumor marker CA 125 was detected in the sera of patients with epithelial ovarian cancer using high levels of labeled monoclonal antibodies. The paper lists the tissues in which CA 125 was found both in normal conditions and during tumor degeneration. Body fluids which normally contain CA 125 levels are presented, as well as various factors responsible for elevated CA 125 levels in the serum, including physiological conditions, benign disorders and malignant diseases. The relationship between the tissue and serum CA 125 levels is explained. Clinical application of serum CA 125 assay to gynecologic diagnosis, disease monitoring and prognosis in ovarian cancer is described, as well as its potential use in patients with endometrial cancer and endometriosis. Finally, different CA 125 assays are presented, as well as the limitations of these methods.