BACKGROUND: Fine particulate matter has been linked to allergies by experimental and epidemiological data having used aggregated data or concentrations provided by fixed-site monitoring stations, which may have led to misclassification of individual exposure to air pollution. METHODS: A semi-individual design was employed to relate individual data on asthma and allergy of 5338 school children (10.4 +/- 0.7 years) attending 108 randomly chosen schools in 6 French cities to the concentrations of PM2.5 (fine particles with aerodynamic diameter 2.5 microm) assessed in proximity of their homes. Children underwent a medical visit including skin prick test (SPT) to common allergens, exercise-induced bronchial (EIB) reactivity and skin examination for flexural dermatitis. Their parents filled in a standardised health questionnaire. RESULTS: After adjustment for confounders and NO2 as a potential modifier, the odds of suffering from EIB and flexural dermatitis at the period of the survey, past year atopic asthma and SPT positivity to indoor allergens were significantly increased in residential settings with PM2.5 concentrations exceeding 10 microg/m3 (WHO air quality limit values). The relationships were strengthened in long-term residents (current address for at least 8 years). CONCLUSIONS: Findings support the hypothesis that changes in allergy prevalence observed in recent decades might be partly related to interactions between traffic-related air pollution and allergens. Further longitudinal investigations are needed to corroborate such results.