We studied regional characteristics of mortality and meteorological conditions in Seoul and Busan during the extreme heat wave of 1994. We estimated the relationship between EHWS’s fixed criteria and observed deaths during 1991–2005. During the same period, HHWS’s warning criteria and observed excess deaths were calculated and compared to the EHWS’s to test the reliability of the system. Because of increasing urban vulnerability, the application and development of a heat warning system is imperative. Application of HHWS will reduce the urban health risks and provide efficient decision-making for public health officials.