Abstract
Recent studies have reported that the spaceflight environment lengthens leukocyte telomeres. We propose that this baffling finding reflects changes in the composition of leukocyte subsets rather than an actual increase in telomere length within individual leukocytes. Since leukocyte telomere length is associated with aging-related diseases and longevity in humans, it is crucial to understand the underlying factors driving telomere length changes in space.