CA 125 remains the only tumour marker to have any significant impact on the clinical management of epithelial ovarian carcinoma. Its role has developed over the past two decades by validating CA 125 criteria in large numbers of patients against established methods of disease assessment. CA125 has a role in the work-up of a patient with a pelvic mass. Its role in prognosis and screening remains uncertain. It is of value in the assessment of response to chemotherapy and to identify disease progression. When compared with previous methods of determining the onset of disease recurrence the use of CA 125 has been shown to be more sensitive and to be able to detect it at an earlier stage. However, it remains unknown as to whether the earlier introduction of chemotherapy actually improves survival. Until the results of an MRC / EORTC trial are available regular CA 125 monitoring during follow-up cannot be recommended. Once recurrence is suspected precise criteria based on CA125 can reliably confirm this. With this developing role it is likely that CA 125 will be of even more importance to the management of epithelial ovarian carcinoma in the future.