Background and Purpose-Accumulating evidence suggests that telomere length is a maker for biological aging of the cardiovascular system. Whether stroke is associated with accelerated biological aging as measured by telomere length has not been conclusively demonstrated. Our aim was to determine whether mean leukocyte telomere length is a predictor for the development of stroke. Methods-The relative telomere length of leukocytes was determined by quantitative polymerase chain reaction in 1309 stroke patients and 1309 age- and sex-matched control subjects as well as 858 stroke patients followed prospectively for 5 years. For each measure, the study sample was divided into quartiles. The associations between the telomere length and risk of stroke as well as poststroke adverse outcomes were determined. Results-Mean telomere length was significantly shorter in stroke patients than in control subjects. Shorter telomere length levels were directly associated with a higher risk of stroke in the case/control sample. As compared with the fourth (longest) quartile, the odd ratios [OR] (and 95% confidence intervals [CI]) for ischemic stroke risk were as follows: third quartile, 1.37 (1.04-1.82); second quartile, 1.53 (1.17-2.02); and first quartile, 2.12 (1.62-2.77). Follow-up of the patients from the prospective cohort also showed that shorter telomere length levels were associated with mortality from all causes but not with recurrence of stroke. Conclusions-Shorter telomere length was associated with ischemic stroke and was a strong predictor of poststroke death. (Stroke. 2012; 43: 658-663.)