<p>The relationship between diurnal temperature range (DTR) and daily mortality from 2005 to 2020 is examined for seven large metropolitan areas in Virginia using distributed lag non-linear models that control for temperature and humidity. The relative risk of mortality increases for very high DTR, and there is a short lag effect of several days. High risk DTR days are rare, typically occurring less than 1% of the time at most locations. These days primarily occur in spring and are characterized by high pressure and low humidity that allow overnight temperatures to drop substantially. High DTR days are often associated with elevated respiratory and cardiovascular mortality and tend to impact the elderly. The similarity between the mortality response to high DTR and that of high temperatures alone, coupled with the lack of clear physiological underpinnings, challenges the notion that these responses are independent. The prevalent spring peak in dangerous DTR days suggests that lack of acclimatization to hot and humid conditions may impose strain on the cardiovascular and respiratory systems.</p>