Telomerase maintains telomere at the end of chromosome and stabilizes chromosome. It is thought to have important roles in cancer progression and cell immortality. We evaluated the role of telomerase expression in colorectal carcinogenesis. Materials included 13 colonic adenomas, 9 early colorectal cancers, 32 advanced colorectal cancers, 5 metastatic tumors, and 30 non-cancerous colon mucosas. The telomerase activity was analyzed using TRAP-eze (Oncor Inc.) for a semi-quantitative method. The positive rate of telomerase activity was 13.3% in non-cancerous colonic mucosa, 15.4% in colonic adenomas, 77.8% in early colorectal cancers, 93.8% in advanced colorectal cancers, and 100% in metastatic tumors; the mean value was 18.0, 29.9, 65.8, 97.0 and 161.3. The correlation between telomerase activity and tumor size, histologic type, or depth of invasion was noted. Sensitivity, specificity and accuracy were on the order of 89%, 98% and 93% at the cut-off level as two times the mean value of non-cancerous mucosa. Telomerase had an important role in carcinogenesis, and progression of colorectal cancer, and it was suggested to be useful for a tumor marker.