Exposure to extremely hot weather has been associated with increased mortality. Temporal Synoptic Index is an effective method used to analyze the relationship between mortality and combined weather factors. The aim of this study is to examine the short-term effect of ambient heat on mortality in Sydney during the warmest 6-month period (October-March) for the years 1993-2001. Eleven synoptic categories were related to daily mortality rates in Sydney. Two distinctive warm categories were associated with significantly higher mortality rates. Hot, dry and relatively rare Synoptic Category 7 (SC7) days showed the highest daily mortality rates, followed by warm and humid SC3 days, which occurred more frequently. Increased mortality was more pronounced among the elderly population, and gender-stratified analysis showed women to be more vulnerable. Mortality on the day of the weather event was higher than 1 or 2 days after the adverse synoptic situation. Ozone and particulate matter smaller than 10 microm were found at high concentrations in SC3 and SC7, respectively, but their impact on mortality was not clear. The population of Sydney was found to be vulnerable to high temperatures, with a lower susceptibility than those of some cities in the USA and Europe.