Two siblings, 16 months and 5 years of age, came into contact with the urticating hairs (setae) of oak processionary caterpillars, the larvae of Thaumetopoea processionea L., when the family moved to a suburb of Vienna, where mass gradation of T. processionea had started the year before. The setae were being spread by the wind from an infested oak tree in the neighbourhood. Both children repeatedly suffered bouts of dermatitis during the 10 weeks of the larval development. Owing to the fact that T. processionea often infests oak trees, whether isolated or at the edges of forests, there is a high likelihood of people being affected. Children frequently explore their surroundings and are at an even greater risk of developing lepidopterism. Caterpillar dermatitis should therefore be taken into consideration in the differential diagnosis of a pruritic rash in infants from regions with caterpillar-infested trees, especially during the larval development of T. processionea.