OBJECTIVE To investigate the association of the micronucleus (MN) formation in lymphocytes from patients with the malignant degrees of colorectal cancer. METHODS The MN test in capillary blood lymphocytes was conducted in 112 patients randomly selected from in-hospital patients before therapy. Experimental data were analyzed by SPSS (v.10.1) software. RESULTS The differences in the frequency of MN between 7 pathological types of colorectal cancers and controls were statistically significant (P<0.01). The frequency of MN increased with the decrease of the histological differentiation in colorectal cancer, and the statistically significant differences were seen between low differentiation group and the other differentiation groups in colorectal cancers. CONCLUSION There is a significant correlation between MN formation and the malignant degrees of colorectal cancer, and MN formation will be a useful biomarker for the identification of malignant degrees of colorectal cancer before operation or for the screening of high risk subgroup.