In 100 ovarian cancer patients serum CA-125 was monitored during treatment and follow-up to verify the prognostic value of its pre-surgery marker level and the usefulness of the test for disease monitoring. A low pre-operative CA-125 was not associated with improved survival or progression-free survival. A significant survival advantage was evident for patients in whom the marker had decreased during treatment, compared to patients in whom the antigen remained unchanged or increased. In 40 patients CA-125 was measured before second look laparotomy: the marker was negative in 100% of patients in, pathological complete response and in 53.5% of patients with residual disease at second look. A marker increase was evident in 32 of 41 patients with either progressive or recurrent disease, and in 17 patients the CA-125 increase was documented from 1 to 9 months before clinical evidence of progression. The pre-operative level of CA-125 did not appear to be of prognostic value. However, the monitoring of CA-125 during treatment and follow-up can provide a reliable method of assessing response and prognosis. CA-125 monitoring is particularly useful for early detection of recurrence.