Telomerase adds hexameric repeats of 5'-TTAGGG-3' to the ends of chromosomal DNA (telomere) and has been implicated in cell immortalization and cellular senescence. The aim of this study was to measure quantitatively the telomerase activity and human telomerase RNA component (hTR) content in gastric cancer and to examine the relation between these values and histologic factors including Helicobacter pylori as a risk factor for gastric cancer. Telomerase activity was measured by a modified telomeric repeat amplification protocol in cancerous and noncancerous tissues (intestinal metaplasia, chronic gastritis, normal mucosa) from 27 gastric cancer patients; hTR expression was examined by the quantitative reverse transcriptase-polymerase chain reaction using fluorescent probes. Telomerase activity was higher in cancers (total product generated: 33.7) than in noncancerous tissues. Telomerase activity was higher in intestinal metaplasia (16.7) and chronic gastritis (10.6) than in normal mucosa (3.5). In patients with intestinal-type gastric cancer, telomerase activity was higher in intestinal metaplasia with H. pylori infection than in that without infection. hTR expression was not correlated with telomerase activity. H. pylori infection may influence telomerase activity in cancer and noncancerous tissues.