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Background: Epidemiological studies have shown that exposure to ambient particles is associated with increased respiratory symp-toms and exacerbations in asthmatic patients. Ultrafine particles (UFP) may contribute to these negative health effects due to their highly reactive surface area and oxidant capacity. The aim of this study was to test the hypothesis that the allergic inflammation in the lung of allergic asthmatics will be enhanced due to the pre-exposure to UFP. Methods: Fourteen mild allergic asthmatics were exposed for 2 h to a controlled atmosphere of 50 lg/m3 spark-discharge generated carbon black UFP (Ø50 nm) or clean air in a randomized, double-blind crossover design. The expositions were performed with a wash-out period of 30 days under intermittent bicycles ergometer exercise. Eighteen hour after the exposures segmental allergen challenges were performed by bronchoscopy. Forty-two h after the expositions, during a second bronchoscopy, the previously challenged segments were lavaged and BAL-cells were obtained for differentiated FACS analysis and oxygen radical generation by chemiluminescence. Results: The preexposition to ultrafine particles showed only a slight nonsignificant short term effect after 42 h on the allergic inflammation compared to the exposition with clean air. The subgroup of subjects however, which were exposed to ultrafine particles in the first period and to clean air in the second period showed after the allergen challenge in the second period a strong increase in the absolute numbers of eosinophils and oxygen radical generation by BAL-cells. Conclusion of the study: The exposure to ultrafine carbon black particles (UFP) 30 days prior to an allergen challenge had a strong pro-inflammatory effect on the allergic inflammation in allergic asthmatics.