CA 125, a tumor marker has been found to be elevated in malign conditions as well as in some benign conditions like heart failure. Cardiac surgery has been shown to cause a systemic inflammatory response. In this study, we investigated alterations in serum levels of CA 125 during cardiac surgery with cardiopulmonary by-pass. Thirty nine patients with a mean age of 54.7 years who underwent either coronary bypass surgery or valvular heart surgery were prospectively recruited to the study. We measured plasma levels of CA 125 preoperatively and on postoperative days I and 7. Comparing with preoperative values, CA 125 levels were similar on postoperative day 1(5.26 [6.89] U/ml vs 5.74 [4.54] U/ml) but elevated significantly on postoperative day 7 (42.1  U/ml, p< 0.0001) (data in median interquantile range). CA 125 levels were found to be elevated after cardiac surgery. The elevations are more in patients undergoing valvular heart surgery than coronary bypass graft surgery. Although CA 125 can be considered a reliable tumor marker in the diagnosis and follow up of patients with malignant diseases, the presence of a recent cardiac surgery with cardiopulmonary bypass must be taken into account when asking for the cause of elevated CA 125 plasma level.