Myelodysplastic syndrome (MDS) is a diverse group of bone marrow diseases, unified by poor blood counts and a propensity for development of acute leukemia. MDS is most often diagnosed in older adults, arising as part of aging and without a toxic exposure or predisposing medical condition. In rare cases, however, MDS develops as a complication of an inherited bone marrow disease, such as dyskeratosis congenita, which causes a defect in telomere maintenance and have an increased risk of developing MDS and leukemia. Telomeres are caps that protects the ends of chromosomes from deterioration, and are important for preventing premature cellular aging. In preliminary studies in a large international cohort of MDS patients, we found that an unexpectedly high number of MDS patients have inherited abnormalities in genes important for telomere maintenance, that most of these patients were not known to have dyskeratosis congenita, and that their survival after bone marrow transplantation was poor, resulting from an elevated risk of transplant-related complications. Moreover, 3% of patients have rare inherited changes in telomerase genes that are of unknown clinical significance. Our results suggest that inherited variability in telomerase function is important for risk of MDS development, but that current methods for accurately identifying the patients with most risk are inadequate. Therefore, we propose to measure telomere length in the blood cells of MDS patients and correlate with clinical and genetic information. Based on this information, we will perform laboratory analysis of candidate mutations to determine their effect on telomerase function. To complete these studies, we already have samples from more than 1500 MDS patients, collected over 10 years from 130 different institutions, with complete genetic and clinical annotation. In the future, the results of this study may inform the prognosis and choice of treatment for MDS patients, and drive development of better therapies.