Various prognostic factors were studied in 29 patients with stage III or IV ovarian cancer who responded to initial chemotherapy after initial diagnostic surgery. The half-life of CA 125 in serum during initial chemotherapy was the most important prognostic indicator for survival (P less than 0.001) and the chance of achieving complete remission (P = 0.012). A CA 125 half-life of less than 20 days, 20-40 days and greater than 40 days appears to identify patients with a good, intermediate or poor prognosis, the two year actuarial survival being 76%, 48% and 0% respectively. The change of achieving a complete remission was 15% and 67% respectively for patients with a serum CA 125 half-life of greater than 20 or less than 20 days.