Cancer antigen 125 (CA 125), a new ovarian cancer-associated antigen, was studied by radioimmunological determination of serum concentrations in 58 healthy blood donors, in 31 women with benign tumors, and 100 patients with malignant tumors of the ovary. Elevated CA 125 levels were found in 5% of normal controls, in 13% of women with benign tumors, and in 78% of patients with ovarian cancer. After successful antineoplastic treatment, false positive CA 125 values were observed in 4% of tumor-free patients. The incidence of pathological CA 125 serum levels was found to depend on the histogenetic origin of the ovarian tumors and was highest in patients with epithelial serous cystadenocarcinomas (85%). Sequential determinations of CA 125 in 27 patients with ovarian cancer under therapy showed a concordance in 89% of cases between serum concentrations and clinical courses. Elevations of CA 125 were already observed 1-6 months before objective evidence of recurrence. Therefore, the determination of serum CA 125 is recommended in the surveillance of patients with ovarian cancer.