CA 125, which is an antigenic determinant expressed by many epithelial ovarian cancers, is measured in serum using a solid phase immunoradiometric assay. Sera from 55 patients who were in clinical remission and underwent a second-look operation to assess disease status after chemotherapy were studied prospectively. All patients had the CA 125 assay performed within one week before their second-look operation. Twenty-four patients (44%) had no histologic or cytologic evidence of disease, seven patients (13%) had microscopic disease, 13 patients (24%) had disease measuring 1 mm to 1.5 cm, and ten patients (18%) had disease greater than or equal to 1.5 cm in maximum tumor dimension. None of the 24 patients with a negative second-look operation had a positive CA 125 antigen level (greater than or equal to 35 U/mL), compared with six of 20 patients (30%) with less than 1.5 cm disease, and six of ten (60%) with greater than or equal to 1.5 cm disease (P less than .0001). All 12 patients with an elevated CA 125 antigen level (greater than or equal to 35 U/mL) had disease discovered at their second-look operation. Thus, in this setting the predictive value of a positive CA 125 titer (greater than or equal to 35 U/mL) was 100%. The predictive value of a negative CA 125 antigen level (less than 35 U/mL) was 56%, ie, the test did not exclude the presence of disease in 44% of patients with a positive second look. The maximum tumor size associated with at least one prior negative antigen level was 1.9 cm.(ABSTRACT TRUNCATED AT 250 WORDS)