It is well established that mortality rates are higher during the winter in the temperate regions of the Northern Hemisphere, and that extreme temperatures during the summer can lead to spikes in mortality. Together, these two phenomena result in a U-shaped relationship between daily mortality and temperature. The shape and position of the U vary by region and average temperature, indicating that populations adapt to their local climate. In the coldest cities, the increase in mortality at low temperatures is relatively moderate, while the increase in mortality at high temperatures is relatively pronounced. Conversely, in the hottest cities, the cold-mortality relationship is stronger than the heat-mortality relationship. While global warming due to anthropogenic greenhouse gas emissions is virtually certain for the coming decades, it is important to examine its potential health impacts. It is also important to understand whether winter mortality is likely to decrease as a result of global warming. This will have significant implications for adapting public health responses. Surprisingly, based on a review of the literature, we conclude that it seems unlikely that winter mortality will decrease as temperatures rise. (R.A.)