Serum CA-125 was determined in 60 patients with variously extended ovarian cancer and monitored during and after chemotherapy. The study indicates that: 1) prechemotherapy CA-125 shows the presence of an active disease with an accuracy greater than 85%. The antigen is elevated in 97% of patients with greater than 2 cm disease, but sensitivity is low (67%) in patients with minimal residual disease (less than 2 cm); 2) changes in CA-125 correspond well with the response to chemotherapy. CA-125 becomes negative in every patient having clinical CR and increases in every patient with progressive disease. These changes can indicate the type of response some months ahead of time; 3) CA-125 indicates in advance the recurrence of the disease after an objective remission: 4) In the conditions studied CA-125 basal levels do not seem to have prognostic value as regarding either response or survival.