Cities are well known to be hotter than the rural areas that surround them; this phenomenon is called the urban heat island.  Heat waves are excessively hot periods during which the air temperatures of both urban and rural areas increase significantly.  However, whether urban and rural temperatures respond in the same way to heat waves remains a critical unanswered question.  In this study, a combination of observational and modeling analyses indicates synergies between urban heat islands and heat waves.  That is, not only do heat waves increase the ambient temperatures, but they also intensify the difference between urban and rural temperatures.  As a result, the added heat stress in cities will be even higher than the sum of the background urban heat island effect and the heat wave effect.  Results presented here also attribute this added impact of heat waves on urban areas to the lack of surface moisture in urban areas and the low wind speed associated with heat waves.  Given that heat waves are projected to become more frequent and that urban populations are substantially increasing, these findings underline the serious heat-related health risks facing urban residents in the twenty-first century.  Adaptation and mitigation strategies will require joint efforts to reinvent the city, allowing for more green spaces and lesser disruption of the natural water cycle.