CA 125 was measured in serum samples from 81 patients with a diagnosis of epithelial ovarian cancer. The samples were drawn prior to second look laparotomy performed in patients with complete or partial clinical remission after finishing cytostatic or radiation treatment. The positive and negative predictive value of the CA 125 measurements was 100% and 64%, respectively; the sensitivity and specificity were found to be 44% and 100%, respectively. At the second look operation no evidence of disease was observed in 40 patients, microscopic tumor was found in 10, and macroscopic tumor in 31 patients. All 81 patients had a gynecologic examination under anesthesia prior to the second look operation. Measurement of CA 125 was found superior to this examination with respect to positive and negative predictive value as well as to sensitivity and specificity. It is concluded that CA 125 values above 35 U/ml without exception announced residual tumor. Therefore, in cases with elevated CA 125 values, gynecologic examination under anesthesia or explorative laparotomy gave no further information concerning the result of treatment. CA 125 levels below 35 U/ml were without predictive value owing to a high number of false negative results.