Population projections of the aging global society and its fiscal and social impact have depended on assumptions regarding the human life span. Until now, the assumption that the maximum human life span is fixed has been justified. Recent advances in cell biology, genetics, and our understanding of the cellular processes that underlie aging, however, have shown that this assumption is invalid in a number of animal models and suggest that this assumption may become invalid for humans as well. In vitro alteration of telomeres affects cellular senescence, and in vivo manipulation of genes and diet can increase maximum life span in animal models if these discoveries are extended to humans. We may soon be able to extend the maximum human life span and postpone or prevent the onset of diseases associated with aging. Such a possibility requires that we recognize a growing uncertainty in any attempt to project international health care costs into the next few decades. The costs may be significantly lower than projections, if life span increases and age-related disabilities are postponed or less severe, or perhaps higher, if life span increases without altering the onset and severity of disability. An appropriate uncertainty regarding the human life span undermines any attempt to accurately predict health costs in the next century.