An increasing number of cities subject to hazardous summer weather in the United States and Canada have begun to develop and implement hot weather response plans to prevent heat-related illnesses and deaths. In this study we focus on heat-mortality relationships in Toronto, Ontario, between 1980 and 1996 and evaluate the potential effectiveness of the city's interim hot weather-health response plan. Using two heat stress indexes--humidex and apparent temperature--we identified excess mortality associated with hot and humid weather and then estimated excess deaths for hot and cool summers. Mortality rates for all ages and for > 64 years age groups rose with increasing humidex and apparent temperature, with no significant increase for the population < 65 years. Excess mortality occurred as low as the 30-35 degrees C humidex range, which is below the 40 degrees C humidex used to issue a heat warning under Toronto's interim hot weather response plan. During a hot summer (such as 1988 or 1995), 32 excess deaths would be expected, whereas 34 fewer deaths than baseline levels would be expected during a cool summer like 1982 or 1992. Days with high humidex levels occur infrequently in Toronto, and thus exposure is limited under current climatic conditions. In the event of a warming climate, more days with dangerously high humidex levels are likely to occur, and summer deaths are expected to increase. Toronto's hot weather health-response plan is an important early step for adaptation to climate change.