BACKGROUND: Concern over the adverse effects of heat on human health has led to numerous studies assessing the relationship between heat and mortality. Few studies have quantified the impact of heat on morbidity, including ambulance response calls. This study describes the association between temperature and ambulance response calls for heat-related illness (HRI) in Toronto, Ontario, Canada during the summer of 2005. METHODS: Data sources included daily temperature, relative humidity and humidex information from Environment Canada, and Medical Priority Dispatch System data from Toronto Emergency Medical Services. Time series and regression analyses were used to examine the relationship between daily temperature and ambulance response calls for HRI during the summer (1 June to 31 August) of 2005. RESULTS: In 2005, there were 201 ambulance response calls for HRI. On average, for every one degree increase in maximum temperature ( degrees C) there was a 29% increase in ambulance response calls for HRI (p<0.0001). For every one degree increase in mean temperature ( degrees C) there was a 32% increase in ambulance response calls for HRI (p<0.0001). CONCLUSIONS: Given these associations, we urge further exploration of ambulance response calls as a source of HRI morbidity data particularly given the increasing health concerns associated with climate change.