Introduction: Between 2008 and 2012, a measles outbreak occurred in mainland France, with more than 22,000 cases reported through the mandatory reporting system (DO), which has served as the basis for epidemiological surveillance of measles since 2005. Other data sources can be used, such as SOS Médecins (SOSM) data, which are integrated into the SurSaUD syndromic surveillance system of the French Institute for Public Health Surveillance. The objective of this study was to evaluate the ability of indicators derived from SOSM data to detect and respond to a measles outbreak. Method: A retrospective study covering the period from 2008 to 2012 identified indicators based on SOSM call reasons and diagnoses and validated them using the DO’s case definition for measles. The EARS control chart method was then applied to the SOSM and DO data to assess the ability of SOSM indicators to identify the onset of an outbreak. Results: Clinical measles diagnoses were selected and validated as the primary SOSM indicator for monitoring a measles outbreak and allow for the identification of the onset of the outbreak at the national level and in regions earlier than the DO. Conclusion: SOSM data enable the identification of the onset of a measles outbreak and the monitoring of its dynamics. The added value of this source lies in the responsiveness of the monitoring, with daily analysis of the epidemic’s progression at the national and regional levels. The approach used in this study could be adapted for other diseases. (R.A.)