Telomerase, an enzyme associated with cellular immortality, is expressed by malignant tumor and stem cells, especially germ cells. Normal somatic cells, however, usually do not express telomerase. In the malignant tumor, deregulation of telomerase is thought to facilitate tumorigenesis and cellular immortality by providing cancer cells unlimited proliferation capacity. We investigated the relationship between proliferation activity and in situ expression of the telomerase RNA component (human telomerase RNA component, hTERC). In addition, in situ hybridization of the telomerase-associated proteins (telomerase-associated protein 1, TEP1; human telomerase reverse transcriptase, TERT), and MIB-1 immunohistochemistry for proliferation activity were performed, using the malignant tumors of adenocarcinoma, squamous cell carcinoma, and malignant lymphoma, and somatic tissues of testis, endometrium, stomach, skin, and lymph nodes. In the somatic tissues, the stem cells expressed telomerase-associated RNA, but no proliferation activity. When the proliferation activity of the stem cells increased, however, the telomerase-associated expressions decreased. In the malignant tumors, both proliferation activity and expression of the telomerase-associated RNA significantly increased. Deregulation of telomerase, in addition to proliferation activity, is associated with tumorigenesis.