AIM: To evaluate the reverse transcriptase-PCR assay and multiple sampling for detection of cytokeratin-positive cells in peripheral blood of colorectal carcinoma patients and to investigate the clinical significance of micrometastasis in peripheral blood. METHODS: The expression of CK20 mRNA by RT-PCR was investigated in bone marrow, portal vein and peripheral blood in 58 colorectal cancer patients and 12 controls without known cancer. The peripheral blood was sampled twice at intervals of 3 d before operation. All the patients were followed up for one year. RESULTS: There was no positive expression of CK20mRNA in 12 volunteers. The positive expression of CK20mRNA was 77.6% (45/58) in bone marrow, and that in portal vein was 74.1% (43/58) of colorectal carcinoma patients. The positive expression of CK20mRNA cells in peripheral blood rose from 44.8% (26/58) to 69.0% (40/58) (P<0.01). The total positivity of CK20mRNA expression in peripheral blood was similar to the positivity of CK20mRNA in bone marrow and portal vein. The positive rates became higher in later clinical stages than in early stages. The CK20mRNA positive patients had a higher relapse rate within one year than the CK20mRNA negative patients. CONCLUSION: Multiple blood sampling can increase the detection of tumor cells in peripheral blood by RT-PCR for CK20mRNA in colorectal carcinoma patients and it is as sensitive and specific as that of bone marrow and portal vein. This technique may be reliable and convenient to diagnose micrometastasis of colorectal carcinoma and has an important significance in determining the prognosis of cancer patients. AIM: To evaluate the reverse transcriptase-PCR assay and multiple sampling for detection of cytokeratin-positive cells in peripheral blood of colorectal carcinoma patients and to investigate the clinical significance of micrometastasis in peripheral blood. METHODS: The expression of CK20 mRNA by RT-PCR was investigated in bone marrow, portal vein and peripheral blood in 58 colorectal cancer patients and 12 controls without known cancer. The peripheral blood was sampled twice at intervals of 3 days before operation. All the patients were followed up for one year. RESULTS: There The positive expression of CK20 mRNA in 12 volunteers. The positive expression of CK20 mRNA was 77.6% (45/58) in bone marrow, and that in portal vein was 74.1% (43/58) of colorectal carcinoma patients. The positive expression of CK20 mRNA cells in peripheral blood rose from 44.8% (26/58) to 69.0% (40/58) (P <0.01). The total positivity of CK20 mRNA expression in peripheral blood was similar to the positivity of CK20mR The positive rates became higher in later clinical stages than in early stages. The CK20 mRNA positive patients had a higher relapse rate within one year than the CK20 mRNA negative patients. CONCLUSION: Multiple blood sampling can increase the detection of tumor cells in peripheral blood by RT-PCR for CK20 mRNA in colorectal carcinoma patients and it is as sensitive and specific as that of bone marrow and portal vein. This technique may be reliable and convenient to diagnose micrometastasis of colorectal carcinoma and has an important significance in determining the prognosis of cancer patients.