Telomerase activity has been found in most cancer cells, but not in the majority of normal differentiated tissues. Therefore, telomerase has been considered a relatively selective and widely expressed tumor marker to be used as a diagnostic tool, and in some cases, as a potential prognostic indicator. Telomerase activity can also be used to evaluate chemosensitivity of neoplastic cells obtained from cancer patients, by measuring residual telomerase activity after drug treatment. Finally, telomerase has been considered to represent a suitable target for designing new anticancer strategies. This review focuses on present and future clinical applications of telomerase studies in cancer management. Copyright 2000 Harcourt Publishers Ltd.