Background. The winter outreach program for homeless individuals, which runs annually from November 1 to March 31, aims to reduce the risk of mortality and morbidity among this population during periods of cold or extreme cold weather. A winter surveillance program monitoring the health effects of cold weather on this population was proposed in the Île-de-France region to assess the adequacy of the measures taken and to alert public authorities in the event of an unusual increase. The objective of the study was to develop an indicator for this surveillance, based on the activity of the region’s emergency departments participating in the Organized Coordinated Emergency Surveillance Network (Oscour®). Methods. Information from the electronic medical records of 11 emergency departments in the Île-de-France region during the winter of 2007–2008 was used to determine homeless status and to confirm diagnoses made during visits selected from the Oscour® database based on a diagnostic code or reason for presentation suggesting hypothermia, frostbite, or chilblains. The indicator was constructed based on the maximization of three criteria concerning positive predictive value, the proportion of homeless individuals, and the number of relevant emergency department visits. A Shewhart control chart was applied to the indicator over four winter seasons between 2005 and 2009 in Paris and its inner suburbs to detect instances where statistical thresholds were exceeded, which could signal a need to adjust the system. Results. Two hundred sixteen records were reviewed. The indicator “number of emergency department visits by individuals aged 15 to 69 with a diagnostic code or reason for presentation suggesting hypothermia,” which had a positive predictive value of nearly 85% and identified 61.7% of homeless individuals, was developed. During the winter of 2008–2009, the statistical alert threshold was exceeded in December during the first cold snap, and again in early January during the severe cold snap. Conclusion. The results demonstrated the value of monitoring this health indicator for adapting the system established as part of the winter mobilization effort for homeless individuals.