© 2016 by The University of Chicago. All rights reserved. This paper examines the temperature-mortality relationship over the course of the twentieth-century United States both for its own interest and to identify potentially useful adaptations for coming decades. There are three primary findings. First, the mortality impact of days with mean temperature exceeding 807F declined by 75 percent. Almost the entire decline occurred after 1960. Second, the diffusion of residential air conditioning explains essentially the entire decline in hot day–related fatalities. Third, using Dubin and McFadden’s discrete continuous model, the present value of US consumer surplus from the introduction of residential air conditioning is estimated to be $85– $185 billion (2012 dollars).