Introduction - Heat waves often have significant health consequences. Prevention plans are developed to mitigate their effects, but no matter how effective they may be, they cannot eliminate all the consequences. This study compares different medical conditions and age groups during and outside of heat wave alerts in the summer of 2006, based on activity data from 49 emergency departments in France. Method - The data used are sent daily by the emergency departments to the InVS via the Internet using FTP (File Transfer Protocol). Various variables are collected, such as age, medical diagnosis coded according to ICD-10, and discharge destination. The comparison of daily averages for the different variables and across different age groups for the two periods is performed using a Student’s t-test. Results - A significant increase in visits for hyperthermia, fainting, dehydration, and hyponatremia is observed among the elderly. The number of daily visits did not increase for the general population but increased significantly among older adults. These initial results indicate that it is necessary to continue and adapt prevention efforts for the population most affected by heat. (R.A.)