Background: Exposure of children to pertussis antigens caused by infection or vaccination with whole-cell pertussis vaccine may increase the serum IgE level and predispose to sensitization to the prevalent environmental allergens. Acellular pertussis vaccine (APV) that may be given to adults may have a similar effect. Objective: The purpose of this study was to determine whether APV will cause an increase in environmental sensitization measured by an increase of serum-specific IgE to the allergens to which adults are exposed during the vaccination period. Methods: One hundred adult hospital employees were randomized to receive either a 2-component APV composed of pertussis toxin and filamentous hemagglutinin or a meningococcal vaccine as a control. Serum-specific IgE level to 2 indoor allergens, cat and dust mite, and 2 outdoor allergens prevalent during the immunization season, Alternaria species and ragweed, was measured by an RIA on sera collected before and 1 month after vaccination. Results: The group that received the APV had no significant change in their serum-specific IgE levels to cat, dust, Alternaria species, or ragweed 1 month after vaccination. Conclusion: A 2-component APV did not predispose to an increase of allergen-specific IgE in an adult population. (J Allergy Clin Immunol 2000;105:170-5.)