The maintenance of the length of chromosomes (telomeres) is the prerequisite of unlimited malignant growth of cells. In normal cells the telomeres will be shortened by each cell division finally leading to cell senescence. Unlimited growing cells possess an enzyme, the telomerase, which is able to maintain the length of the telomeres by de novo synthesis of the telomeric repeats. In consequence the telomerase is detectable in nearly all malignant tumours and cell lines. However, in addition to malignant cells some type of non-malignant cells such as germ cells, activated lymphocytes and basal epithelial cells contain significant amounts of telomerase activity. Therefore, the detection of telomerase activity by extractive procedures such as TRAP assay is not sufficient to determine malignant nature of tissue samples.