Recent evidence indicates that telomerase activity may be necessary for cell immortality, which is required for the sustained and indefinite growth of most malignant cells. We analyzed telomerase activity in gastric and colorectal cancers and in gastric and colorectal precancerous lesions to determine whether malignant progression depends on the activation of telomerase and at what stage of carcinogenesis cells have detectable telomerase activity. Telomerase activity was measured by the telomeric repeat amplification protocol assay and was detected in 17 (85%) of 20 primary gastric carcinoma tissues and in 19 (95%) of 20 primary colorectal carcinomas, regardless of tumor staging and histological types. All nodal metastases, peritoneal metastases, and a recurrent gastric cancer tumor were positive. All cell lines established from gastric and colorectal cancers contained telomerase activity. In precancerous lesions, 10 (100%) of 10 colorectal tubular adenomas were telomerase positive, in addition to 3 (23%) of 13 gastric intestinal metaplasias and 1 (50%) of 2 gastric adenomas, whereas the corresponding gastric normal mucosas as well as colorectal mucosas were negative. These results indicate overall that reactivation of telomerase may occur at an early stage of carcinogenesis and may correlate well with malignant progression of gastric cancer. Telomerase activity thus may serve as a powerful additional tool for cancer diagnosis.