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Notes: Background  Thaumetopoea processionea L. (order Lepidoptera), the oak processionary moth, is found in oak forests in most European countries. Its third to sixth larval instars are armed with poisonous hairs (setae) containing an urticating toxin (thaumetopoein) potentially harmful to humans. Because T. processionea infests trees at the edges of forests or standing alone people frequently come into contact with its setae. In the woodland bordering on the western suburbs of Vienna conditions favouring its increase have led to frequent outbreaks of lepidopterism.Objectives  To determine the incidence of lepidopterism in a suburban environment with three separate caterpillar-infested oak trees and to ascertain the frequency of the various symptoms of lepidopterism and the manner of contact with setae.Methods  We conducted a telephone survey of all the households/institutions located within 500 m of the infested trees. To gain more information on patients' symptoms and on situations likely to lead to increased contact with setae we asked those who reported cutaneous reactions to complete a questionnaire. As part of the environmental study we described the outbreak site, examined patients and, with tape-strip samples taken from the surface of the soil, looked for setae persisting in the environment.Results  Of 1025 people surveyed 57 (5·6%) reported one or more symptoms of lepidopterism: 55 (96%) reported pruritus, 54 (95%) dermatitis, eight (14%) conjunctivitis, eight (14%) pharyngitis and two (4%) respiratory distress. The questionnaire was returned by 37 (69%) of the individuals with dermatitis. Of those, 16% had reacted with weal formation, 49% with papular rash and 22% with toxic irritant dermatitis. In 13% of respondents it was not possible to define the reaction. The risk factor analysis showed that airborne contamination was the most important cause: 97% of people had frequently passed an infested tree, 57% lived near a tree (in a neighbouring garden) and 32% had a tree in their own garden. Direct contact with larvae was of minor importance (38%). In four of the tape-strip samples intact setae were identified 1 year after the infestation.Conclusions  Lepidopterism caused by T. processionea is a public health problem of increasing significance. In years with outbreaks of the pest it can reach epidemic proportions in communities located near infested trees. Contact with airborne setae was mainly responsible for the occurrence of the disease.