In human somatic cells without the activity of telomerase, the ends of chromosomes consisting of the telomeric repeats TTAGGG progressively erode with each cell division. In germline and immortal cells telomerase activity maintains telomere length and thus compensates for the 'end-replication problem'. Progressive telomere shortening and reactivation of telomerase activity have been considered to be one of the key mechanisms in cellular senescence and immortalization. It has been shown that while most somatic cells do not have detectable telomerase activity, almost all cancers do have telomerase activity. Thus, detection of telomerase activity may have utility in the early diagnosis of cancer and may be a new target for therapeutic intervention. However, there is recent evidence that some cells of renewal tissues, such as hematopoietic cells and basal cells of the epidermis, have detectable telomerase activity. In the present study, we report detectable telomerase activity in normal human intestinal mucosa. This activity is localized to the lower third of each crypt and may be derived from intestinal stem cells. Since intestinal telomeric repeats are shorter in adults when compared to children, the telomerase activity in the intestine is insufficient to maintain telomere length but may be sufficient to provide extended proliferative capacity for such renewal tissues.