Published cost-effectiveness analyses may overstate the cost-effectiveness ratio of preventive care if they do not explicitly model the costs of the last year of life, which is postponed by prevention. To determine the degree of overestimation, the authors built a statistical model using Medicare expenditure data on survivors and decedents. The model shows that the cost-effectiveness ratio of prevention may decrease by up to US$ 11,000 per quality-adjusted life year saved when expenditure data on the last year life are used. The model is able to explain more than half of the median cost increase of published cost-effectiveness analyses on clinical preventive services.