A cluster of cases of idiopathic palmar-plantar hidradenitis occurred among children and adolescents at a recreation center in Longueville-sur-Scie in late July 2012. Idiopathic palmo-plantar hidradenitis is a rare skin disease whose pathogenesis remains unknown. It is a localized inflammatory reaction characterized by an infiltration of inflammatory cells around the sweat glands and sweat ducts, primarily located on the extremities (palms and soles). A descriptive investigation was conducted by the Cire Normandie to determine the cause of this outbreak. The factors responsible for the onset of these cases were not formally identified, but they likely included the successive performance of physical activities (swimming and walking). The role of visiting a municipal swimming pool as the sole trigger for the illness was ruled out based on the following arguments: no cases were found among people who visited the pool during the period when cases appeared, aside from those at the recreation center; normal physicochemical and microbiological analyses of the pool water. (R.A.)