The effect of injection site on reactogenicity and specific immunogenicity was assessed in participants in a pertussis vaccine efficacy trial. The percent of DTwP and DTaP recipients with any reaction was slightly lower in subjects injected in the buttock compared with those injected in the thigh. This finding was most common in DTwP recipients. Geometric mean antibody values to pertactin, filamentous hemagglutinin and fimbriae (Bordetella pertussis antigens) were lower in DTaP vaccinees when buttock was utilized as the injection site. Our findings present evidence against the use of the buttock as the site of immunization for DTaP vaccines since the benefit with regards to reactogenicity is minimal and the immunologic response to an important antigen of B. pertussis, pertactin, is decreased. (c) 2005 Elsevier Ltd. All rights reserved.