Telomerase activity is generally absent in primary cell cultures and normal tissues. Telomerase is known to be induced upon immortalization or malignant transformation of human cells. In the present study, we analyzed both telomere length and telomerase activity in biopsy samples from mucosa undergoing metaplasia, adenoma and cancer of the stomach. We attempted to estimate the correlation between telomerase activity and telomere length in these tissues. Telomerase activity was estimated using the telomeric repeat amplification protocol and telomere length by Southern blot analysis. Extracts were defined as telomerase-negative when the signals were less intense than those for 10(2) KATO-III cells (positive control). We detected telomerase activity in 15%, 45% and 89% of the examined cases of intestinal metaplasia, adenoma and gastric cancer respectively. However, telomere length in the gastric mucosa became reduced as the mucosa underwent metaplasia and developed into adenoma. Gastric cancers showed a broad range of telomere length among cases. However, gastric adenomas showed the shortest telomere length. These results suggest that telomerase is expressed during the early phase (intestinal metaplasia through adenoma) of gastric carcinogenesis, although the activity at that stage is not high enough to fully restore the reduced telomeric DNA.