Despite curative tumor resection, about 30%-50% of patients with locally advanced gastrointestinal (GI) carcinoma develop tumor recurrence which may be caused by pre- or intraoperative tumor cell dissemination. We examined the combination of optimized density gradient centrifugation with a CK-20 reverse transcriptase-polymerase chain reaction to detect and quantify circulating tumor cells in peripheral blood. Peripheral venous blood (20 ml) of patients with GI carcinomas was collected during primary tumor staging before and after the endoscopy procedure. CK-20 expression in peripheral venous blood was found in 22 of 82 patients (26.8%) with a nonsignificant difference between the upper GI tract (23.9%) and the lower GI tract (30.5%). The correlation with clinical outcome (24-month-survival) revealed a significantly worse prognosis (p < 0.05) of CK-20-positive patients with carcinoma of the upper GI tract and a trend toward a worse prognosis for patients with carcinoma of the lower GI tract. Quantification of CK-20 expression in peripheral blood showed a significantly higher circulating CK-20 copy number (median: 2816) in patients with metastatic tumors than in those with non-metastatic tumors (median: 983) (p < 0.05). For a subset of 42 primarily operated patients, we correlated the detection rate with UICC (International Union Against Cancer) staging categories. In contrast to the upper GI tract, the detection rate of patients with carcinoma of the lower GI tract showed a trend toward tumor size (pT) and a significant correlation with the presence of distant metastases (pM) (p < 0.01) and the postoperative residual tumor status (R) (p < 0.01). The endoscopy procedure did not lead to an increased detection of CK-20 expression.