Hematogenous dissemination is a significant short-coming of colorectal carcinoma treatment. To screen patients with high risk for such blood-borne metastasis, we previously developed a highly sensitive system for the detection of cytokeratin 20 (CK-20) mRNA in blood. For a more practical application, we improved this system by making it quantitative and capable of analyzing peripheral venous blood for the detection of perioperative changes in CK-20 mRNA. CK-20 mRNA was not always detected in the preoperative blood, even in patients in an advanced stage, but it was identified without fail in intra- and post-operative blood. In addition, more copies of CK-20 mRNA were observed in the intra-operative blood than in pre- and post-operative blood. This study suggests that analysis of perioperative changes may provide important information for the precise evaluation of hematogenous dissemination and of the effect of surgical maneuvers on recurrence.