Dyskeratosis congenita (DC) is a premature aging syndrome characterized by progressive bone marrow failure, abnormal skin pigmentation and nail dystrophy. We have recently described an autosomal dominant form of DC (AD DC) in a large three-generation kindred that is due to a mutation in the gene encoding human telomerase RNA (hTR). Importantly, we have noted progressive shortening of telomeres in lymphocytes from the most recent generation, correlating with earlier onset of severe cytopenias in some of these patients. While telomere shortening is a normal consequence of the aging process, DC patients display accelerated telomere shortening in many somatic cell types. Allogeneic hematopoietic stem cell transplant (HSCT) remains the only curative therapy for marrow failure in DC. However, HSCT in DC is generally poorly tolerated and associated with significant morbidity, perhaps as a consequence of increased sensitivity of dividing cells to cytotoxic agents. To test this hypothesis, we characterized lymphocytes from nine AD DC patients and age matched controls that had been placed in long term culture following in vitro exposure to irradiation (137Cs) and varying doses of Taxol. Cell proliferation and viability were quantitated by direct visual counting on a hemocytometer, and flow cytometry was employed to assess apoptosis and cell surface expression of senescent markers. CD57 and CD95, markers of cellular senescence and apoptosis, were significantly upregulated on DC T lymphocytes after two weeks in culture relative to controls. In addition to DC lymphocytes having a decreased proliferative capacity, an increased sensitivity to Taxol was noted, with an average decrease of 21% in cell growth relative to similarly treated control cells. This effect was also noted in irradiated DC cells. Finally, DC lymphocytes displayed an increased apoptotic index in the presence of varying doses of Taxol. These results suggest that telomere shortening may be an important factor in determining cellular tolerance to cytotoxic therapy and support the concept of reduced intensity HSCT regimens in both aged individuals and DC patients.