Telomerase activity and hTERT mRNA expression are upregulated in colorectal cancer. Whether they are inherent in colorectal adenomas, premalignant lesions to cancer, however, remains to be elucidated. We examined telomerase activity by the fluorescence-based telomeric repeat amplification protocol method and analyzed the level of hTERT mRNA by real-time polymerase chain reaction in 74 surgically obtained neoplasms from 29 patients. The specimens were divided into 6 categories according to the criteria of the Vienna Classification. The control comprising 29 non-pathological mucosa were classified into category 1, 6 adenomas indefinite for neoplasia into category 2, 21 non-invasive low grade adenomas into category 3, 23 high grade adenomas or non-invasive carcinomas into category 4, and 15 intramucosal or submucosal carcinomas into category 5. Carcinoma invading beyond the submucosa (9 samples) was referentially subdivided into category 6. Telomerase activity (mean +/- standard error) in 1 categories to 6 were 5.0+/-1.2, 1.8+/-1.7, 4.3+/-1.6, 20.2+/-2.1, 36.4+/-5.5, and 55.5+/-8.2 units/microg protein, respectively. There were no statistical differences between categories 1 and 2, 1 and 3, and 2 and 3. A significant statistical difference in the other two was observed by the multiple comparison test. The mean levels of hTERT mRNA was 103.1+/-102.4, 103.6+/-103.0, 103.6+/-102.9, 103.7+/-102.9, 104.0+/-103.4, and 104.4+/-104.0 copies/microg total RNA, respectively. There was a significant statistical difference only between category 6 and each of the other categories. These results suggest that telomerase activation occurs during the progression from low-grade to high-grade dysplasia in adenomas and increases steadily with the progression of the degree of dysplasia and invasion during colorectal carcinogenesis, and that hTERT mRNA expression is a feature of the late stage development of colorectal cancer.