We evaluated the reactogenicity and immunogenicity of an acellular pertussis vaccine in 24 subjects affected by Down's syndrome and in 10 normal infants. Neither general nor local adverse reactions were observed in either group of subjects. The new acellular vaccine administration elicited protective levels of antibodies in all the subjects with Down's syndrome, although the geometric mean titres of IgG antibodies against Bordetella pertussis in these subjects were significantly lower than in normal controls.