This review describes the physicochemical and biological properties of antigen CA125 and clinical utility of antigen in oncologic diagnostic. CA125 is the most reliable marker for ovarian cancer in clinical practice. Analysis of CA125 values performed by many authors revealed its highest concentrations in serous and endrometrioid histological type. Serial measurements of CA125 are useful in monitoring the response to chemotherapy and follow-up the early detection of recurrent disease of patients with ovarian cancer.