Telomerase is a ribonucleoprotein complex which, by de novo synthesized telomeric TTAGGG repeats, prevents telomere erosion. While telomerase is active in most cancers, conflicting results exist for normal tissues and premalignant lesions. To establish the telomerase status of normal gastrointestinal mucosa and to elucidate whether telomerase up-regulation is an early or late event in carcinogenesis, we determined the telomerase activity of 88 biopsies of normal mucosa from esophagus, stomach, and intestine and compared it with that of 21 samples of premalignant lesions and 6 adenocarcinomas using the telomere-repeat amplification protocol assay. Telomerase was found in all normal tissues, revealing most activity in esophagus (11 samples), followed by intestine (45 samples), and stomach (32 samples). In 53% of the stomach samples, enzyme activity could only be demonstrated when telomerase inhibitors were eliminated by a modified telomerase assay. In the 21 precancerous lesions (5 Barrett's esophagus, 3 stomach intestinal metaplasias, and 13 colorectal adenomas of type I/II dysplasia) a similar or even reduced telomerase activity was seen, while the adenocarcinomas showed high activity. These data demonstrate that telomerase activity is expressed in all epithelia along the gastrointestinal tract, thus confirming our previous hypothesis that telomerase is constitutively expressed in permanently renewing epithelia. Furthermore, activity was not increased in preneoplastic lesions, suggesting that telomerase up-regulation is a late event during carcinogenesis of the esophagus, stomach, and intestine.