BACKGROUND: Heat wave prevention plans are traditionally implemented according to a temperature limit above which mortality begins to rise. Although these prevention plans are obviously designed to avoid deaths, it is also necessary to establish the impact of extreme temperatures on hospital admissions in order to put hospital alert plans into action for dealing with people affected by heat wave victims. METHODS: We used data on daily emergency admissions between May and September, from 1995 to 2000, in the Hospital General Universitario Gregorio Maranon in Madrid. The causes for admission were considered as 'organic' (International Classification of Diseases, ICD-9: 1-799), circulatory (ICD-9: 390-459) and respiratory (ICD-9: 460-519). We stratified them according to the following age groups: all ages, from 0 to 10, 18 to 44, 45 to 64, 65 to 74 and above 75 years. The methodology used was Autorregresive Integrated Moving Average (ARIMA) modelling, including variables related to atmospheric pollution, seasonality and trends. RESULTS: The results show that the temperature above which hospital admissions soar coincides with the temperature limit above which mortality sharply rises, which, in turn, coincides with percentile 95 of the maximum daily temperature series for summer months. The pattern of hospital admissions is completely different from that of mortality. The rise in hospital admissions due to all causes and age groups is clearly smaller than that detected for mortality. Discussion: These results suggest that people die rapidly from circulatory diseases before they can be admitted to hospital. This datum is vital with regard to implementing prevention plans prior to the arrival of the heat wave, if they are to effectively reduce mortality.