The serum levels of antigen CA 125 expressed by epithelial ovarian carcinoma were measured in 27 postmenopausal women with ovarian tumours and in 16 controls. Increased serum levels of CA 125 were found in nine (75%) out of 12 patients with ovarian cancer; in three with stage I disease levels were not elevated. No significant difference was found in the concentration of CA 125 detected in peripheral or ovarian venous blood. Decreased antigen levels were found 6-30 weeks after radical operation and cytostatic chemotherapy in the ovarian cancer group. The results indicate the value of measuring CA 125 as a tumour marker in the follow-up of ovarian cancer.