Plasma levels of CA 125 were determined in 113 patients with ovarian cancer of epithelial origin. Of these, 69 patients had CA 125 measured before the first laparotomy and 84.6% of them had a CA 125 level greater than 35 U/ml. In 87 of the 113 patients whose tumour was producing CA 125, a good correlation was observed between the CA 125 levels and the clinical follow-up: 95.7% of the patients in remission had levels less than 35 U/ml, whereas all the patients with no change or with a progressive disease had levels greater than 35 U/ml. Furthermore in recurrent disease the levels of CA 125 were also increased (greater than 35 U/ml) in 92.3% of the patients. Thus, CA 125 measurements at regular intervals are of great clinical value in following the evolution of a tumour or the success of a therapy, but unfortunately do not allow detection of an ovarian tumour at an early stage.