Telomeres are repeated DNA sequences, positioned at the ends of chromosomes and are essential for the stable maintenance of chromosomes. The telomere length serves as a mitotic clock determining the remaining replicative capacity of the cell. Telomeric sequences are lost during each cell division, leading to a process thought to contribute to senescence and cell death. The enzyme telomerase adds 5'-TTAGGG-3' repeats to the mammalian telomeres and maintains the telomere length. Telomerase is normally inactive in most somatic cells but telomerase activity is observed in malignancies. In this study telomerase activity was analyzed in patients with chronic myeloid leukemia (CML) and lymphoma by PCR and ELISA. This approach combines highly specific amplification of the telomerase-mediated elongation products with nonradioactive detection in a highly sensitive photometric ELISA. The PCR products were also analyzed by Southern blotting. The telomerase-specific PCR products were seperated by electrophoresis and transferred to a nylon membrane with subsequent detection of the biotinylated amplificates. High activity levers were detected in 17 CML ( 34 %) patients. On the other hand, no activity was observed in lymphoma patients. An increase in the shorter telomeric bands was observed in CML patients who displayed a high level of telomerase activity. In contrast to the low enzyme activity, evidence of telomeric repeats were also found in some lymphoma patients by Southern blotting. This may indicate that lymphoma cells may make use of different pathways for maintaining the length of their telomeres.