Objective:To determine whether the decline in CA 125 level after surgery can predict the extent of residual disease.Methods:In a prospective, nonrandomized clinical trial, 27 women with advanced ovarian cancer (stage III or IV) underwent primary or secondary cytoreductive surgery. CA 125 levels were measured before surgery and 3-14 days postoperatively. The mean postoperative decline was compared by size of residual disease.Results:In women with all visible cancer resected, mean (± standard error of the mean) decline in CA 125 was 91 ± 4% (1032 ± 473 U/mL preoperatively to 90 ± 31 U/mL postoperatively). With less than 2.0 cm residual, there was an 85 ± 2% decline in CA 125 after surgery (3061 ± 835 U/mL preoperatively to 456 ± 146 U/mL postoperatively). In patients with residual disease larger than 2.0 cm, the levels dropped 36 ± 10% (2958 ± 1587 to 1904 ± 708 U/mL). If CA 125 declined less than 60% from the preoperative level, sensitivity, specificity, and positive and negative predictive values for residual disease larger than 2.0 cm were all 100%.Conclusion:By using the ratio of CA 125 levels before after cytoreductive surgery, one can predict the likelihood that the patient was left with less than 2.0 cm residual disease. (Obstet Gynecol 1993;81:29-32)