INTRODUCTION: Following the 2003 heat wave, many European countries implemented heat-wave prevention plans. A number of aspects can prove fundamental in determining the effectiveness of such plans, and of these we sought to analyse the criteria used to define threshold temperatures and trigger a higher level of intervention. METHOD: Retrospective study of the days on which heat-wave thresholds were exceeded during the period 1974-2003 was conducted. We compared when and at what level the heat-wave prevention plan would have been activated using a statistical-meteorological criterion (as applied by the Spanish Ministry of Health & Consumer Affairs) versus a temperature-mortality criterion. RESULTS: The number of days on which the threshold was exceeded was far higher when the temperature-mortality criterion was applied. The temperature percentile at which a heat wave occurred was different for each province analysed and was inversely proportional to its respective ageing index. Using both criteria, there was an increase in heat-wave days per decade. CONCLUSION: The establishment of a heat-wave threshold temperature must be based on knowledge of the cause-effect relationship between temperature and the health of a given population. Mortality is an appropriate indicator of population health. The future effects of climate change render it essential for this relationship to be studied on a local scale, so as to enable truly efficient prevention plans to be drawn up.