The effect of abdominal surgery on the serum concentration of the tumour-associated antigen CA 125. The CA 125 assay is used to monitor the course of disease in women with adenocarcinoma of the genital tract. We measured serum CA 125 levels longitudinally in three different groups of patients who had normal serum CA 125 levels (less than or equal to 16 U/ml) before extensive intraperitoneal abdominal surgery (group 1, second-look laparotomy in 28 women with ovarian cancer; group 2, radical hysterectomy in 42 patients with cervical cancer; group 3, 13 men and one woman who had aortic surgery for atherosclerotic occlusive disease or aneurysm formation). Following surgery, rising serum CA 125 levels were observed in 69 out of the 84 patients (82%), irrespective of the primary diagnosis, type of operation or sex. The highest levels were found during the second week after the operation (range 3-336 U/ml) and decreased gradually thereafter, to become normal at 8 weeks after surgery. It was concluded that abdominal surgery interferes with the specificity of CA 125 as a tumour marker during the early postoperative period. 