In a prospective study CA 125 serum levels of 99 patients with ovarian cancer were determined serially. 50 of 55 patients with second-look-laparotomy elevated levels initially had. We were interested in particular in the correlation between CA 125 and the histological findings at second-look operation. In 16 patients there was no residual tumor, CA 125 was in the normal range. In 34 the women tumor tissue was demonstrated; CA 125 was elevated in 14 patients, so there were 20 patients with false negative CA 125 serum levels. No false positive CA 125 values were found. In all patients with elevated tumor marker the residual tumor was histologically confirmed at second look. To verify a complete remission, a second-look operation will have to be performed in spite of a negative tumor marker, since active tumor tissue might still be present. In patients with elevated CA 125 a second-look laparotomy should be avoided at this time.