One goal of the smart growth movement is a more compact urban form, intended to reduce energy use and the cost of moving materials, products, and people. The benefits of compactness are compromised, however, if higher densities and more intense land use create urban heat islands, which increase water and energy use. This study examines the effects of Phoenix's urban heat island on water use by single-family residences, controlling for relevant population and housing attributes. Our statistical analysis demonstrates that increasing daily low temperatures by 1° Fahrenheit is associated with an average monthly increase in water use of 290 gallons for a typical single-family unit. These results suggest that planners should consider effects on water demand as well as other environmental consequences when they evaluate growth strategies, and use incentives to encourage efficiency and sustainability.