The oak processionary caterpillar (Thaumetopoea processionea Lepidoptera) is found in several European countries. It usually lives in oak forests or on single-standing oak trees. The larva of from the 3rd to 6th developmental stage (instar) develops poisonous hair (setae), filled with an urticating toxin that may lead to serious dermatitis, conjunctivitis, and pulmonary affection (summarized as lepidopterism) on contact with the setae. In June 2004 more than 40 people including young children developed symptoms of lepidopterism after resting within 20 m of an infested oak tree. Only a few people had touched the caterpillars. All those with clinical symptoms of lepidopterism could be treated as outpatients, but several needed systemic steroids because of the severity of their complaints. Lepidopterism, an airborne disease caused by the setae of the processionary caterpillar, is a growing public health problem because of the increasing numbers of outbreaks.