PURPOSE: To identify the prognostic factors associated with mortality in heat-related illness. METHODS: Multi-center observational cohort-study in 16 emergency departments (ED) belonging to the teaching hospital network of the Paris area. The cohort comprised all patients admitted to one of the EDs during the August 2003 heat wave in Paris and having a core temperature >38.5 degrees C. Baseline clinical and biological data in ED, patient's course and 1-year survival rate were recorded. Potential prognostic factors associated with death were assessed by Cox proportional-hazards analysis. RESULTS: A total of 1,456 patients were included. Mean age was 79 +/- 19 years. Critically ill conditions were noted in 391 patients (27%), but only 72 (5%) were admitted into an intensive care unit. The survival rate was 57% at 1 year as compared to an expected 90% (P < 0.001). Nine independent prognostic factors were identified: previous treatment with diuretics, living in an institution, age >80 years, cardiac disease, cancer, core temperature >40 degrees C, systolic arterial pressure <100 mmHg, Glasgow coma scale <12 and transportation to hospital by ambulance. We defined three risk groups: low, intermediate and high risk, with a 1-year survival rate of 85, 61 and 18%, respectively. CONCLUSIONS: We observed a low survival rate and developed a risk score based on easily obtained variables that may be useful to clinicians managing casualties from future heat waves.