This paper builds on Barreca et al.’s (2013) finding that over the course of the 20th century the proliferation of residential air conditioning led to a remarkable decline in mortality due to extreme temperature days in the United States. Using panel data on monthly mortality rates of U.S. states and daily temperature variables for over a century (1900-2004) it explores the regional evolution in this relationship and documents two key findings. First, the impact of extreme heat on mortality is notably smaller in states that more frequently experience extreme heat. Second, the difference in the heat-mortality relationship between hot and cold states declined over the period 1900-2004, though it persisted through 2004. For example, the effect of hot days on mortality in cool states over the years 1980-2004, a period when residential air conditioning was widely available, is almost identical to the effect of hot days on mortality in hot states over the years 1900-1939, a period when air conditioning was not available for homes. Continuing differences in the mortality consequences of hot days suggests that health motivated adaptation to climate change may be slow and costly around the world.