Background: Exposure to airborne wheat allergen is a well-known cause of bakers' allergy and asthma. Airborne wheat allergen can be measured by enzyme immunoassays (EIAs) in extracts of inhalable dust samples, but only limited knowledge is available on the size distribution of wheat allergen-carrying particles. Recently, a new sampling medium, porous polyurethane foam, has been introduced for the size-selective sampling of airborne dust in various occupational settings. We investigated the applicability of these foams for size-selective wheat allergen measurements. Methods: Personal and stationary measurements were performed in a flour mill, using respirable and thoracic foams inserted into the conventional IOM inhalable sampler, together with PTFE (Teflon) filters. Foams and filters were eluted and wheat allergen levels determined by human IgG4 inhibition EIA. Results: Wheat allergen levels could be determined in both filter and foam eluates. Inhalable dust levels from filters and foams ranged from 1.4 to 53 mg m-3, and wheat allergen levels from 15 to 580 μg m-3. The allergen was mainly borne on particles with Dae (particle aerodynamic diameter) > 10 μm and particles with 4 μm > Dae ≤ 10 μm, accounting for 54.5-77.5% and 18.9-43.2% of the total allergen yield, respectively. Less than 4% of airborne wheat allergen was carried by particles smaller than 4 μm (respirable fraction). Conclusions: Measurement of wheat allergen in dust fractions trapped in respirable and thoracic foams is technically feasible. Both wheat flour dust and wheat allergen are mainly concentrated in larger particle-size fractions (extrathoracic and tracheobronchial). © 2005 British Occupational Hygiene Society Published by Oxford University Press.