During the summers of 2009 and 2010, an increase in outbreaks of oak processionary caterpillars was recorded around Lake Der. In 2011, treatment was partially applied around the lake to protect local populations. The surveillance program implemented by the Cire Ile-de-France and Champagne-Ardenne aimed to monitor the occurrence of cases (individuals who developed symptoms consistent with exposure to the caterpillars following visits to Lake Der) in order to assess the effectiveness of this measure. Cases were actively collected from healthcare professionals partnering with the initiative and characterized in terms of symptoms and exposure via a self-administered questionnaire provided to patients by these professionals. Pharmacies and general practitioners identified 72 and 79 cases, respectively. Twenty-nine cases returned the questionnaire. All reported skin lesions; 18 cases visited only untreated sites, and seven cases visited treated sites. Forest walks were undertaken by 62% of cases. The results suggest that the treatment was effective in limiting the occurrence of cases. However, a residual health impact was observed. This measure should be repeated for the upcoming summer season and, if possible, expanded, particularly along hiking trails. (R.A.)