Telomerase is a specialized type of reverse transcriptase that catalyzes the synthesis and extension of telomeric DNA. High levels of telomerase activity have been detected in most hepatocellular carcinoma (HCC) tissues; very weak telomerase activity is, however, detected in approximately half of nontumorous chronic liver disease tissues. The purpose of this study was to investigate the possible source of this weak telomerase activity in these tissues using quantitative competitive reverse transcription (RT)-polymerase chain reaction (PCR) and in situ RT-PCR. Competitive RT-PCR indicated that the relative amount of human telomerase RNA (hTR) was significantly higher in chronic hepatitis or liver cirrhosis compared with the normal liver (p < 0.005), and in HCC compared with the normal liver (p < 0.001) and with chronic hepatitis or liver cirrhosis (p < 0.0001). In the normal liver tissue, hTR was detected by in situ RT-PCR in occasional sinusoidal cells and nuclei of occasional hepatocytes. In tumor-free liver or tumor-bearing liver, hTR was detected in sinusoidal cells, infiltrating lymphocytes, occasional proliferative bile ductal epithelial cells, and the nuclei of occasional hepatocytes. In HCC, hTR was detected in nuclei of all HCC cells as an intense signal and in sinusoidal cells. These results indicate that the amount of hTR increases in the nuclei of hepatocytes during hepatocarcinogenesis, and that the cells associated with the weak telomerase activity in approximately half of the nontumorous chronic liver lesions are mainly migrating lymphocytes and sinusoidal cells. (Lab Invest 1999, 79:15-26).