Following a large-scale outbreak of Lithosia quadra caterpillars in August 2006 in the Loire-Atlantique department, the Interregional Epidemiology Unit (CIRE) of the Pays de la Loire region conducted a study aimed at improving knowledge of the health effects associated with contact with this species of caterpillar. A descriptive and qualitative survey was conducted among two populations (members of the public and physicians). The study area was defined by the municipalities from which calls from members of the public were recorded. Data were collected during the study period (August–September 2006) via a semi-structured telephone interview based on a standardized questionnaire exploring health effects and the circumstances of exposure. Of 25 members of the public, 19 had come into contact with a caterpillar (directly and/or indirectly). Among them, 11 presented with clinical symptoms, all of which were cutaneous (notably itching and redness). The median duration of symptoms was less than 24 hours. No hospitalizations were related to the caterpillars. Regarding modes of exposure, 91% [21/23] of individuals reported the presence of caterpillars on house facades and also in their gardens, 61% on roofs [14/23], and 48% inside homes [11/23]. Among the 7 participating physicians, it was not possible to determine whether the clinical signs, all of which were cutaneous (hives, facial edema, maculopapular rash, eczematous rash), were specific to L. quadra or not. Only one case of generalized urticaria was reported. None of the patients seen in the clinic were hospitalized. This study described the health effects of the species L. quadra, which are very poorly documented in the literature. The clinical signs collected appear to indicate mild symptoms that resolve within a few hours. Furthermore, while the described skin manifestations are primarily linked to direct contact between the caterpillar and the skin, it cannot be ruled out that contact with the caterpillar’s filaments may also cause symptoms. (R.A.)