The aim of this study was to examine whether there is a correlation between parental information on the child's history of whooping cough and the presence or absence of serum antibodies against two antigens of Bordetella pertussis, pertussis toxin and filamentous hemagglutinin, in nonvaccinated Swedish children. The parents of 266 Swedish children aged 1 to 4 years answered a questionnaire regarding the child's history of whooping cough, and a serum sample was obtained from the child for determination of IgG, IgM, and IgA antibodies to pertussis toxin and filamentous hemagglutinin. The study was performed from 1984 to 1986, five to seven years after the cessation of general vaccination against pertussis in Sweden; none of the children had received pertussis vaccine. Antibodies to both toxin and filamentous hemagglutinin increased with age. Of the children aged 4 years, 50% had antibodies to both antigens. Of all 266 children, 100 had antibodies to both antigens, 6 to toxin alone, and 49 to filamentous hemagglutinin alone. There was a good correlation between the presence of antibodies and a history of whooping cough. Of 91 children with a history of whooping cough, 77 had antibodies against both antigens and 13 against one antigen; only one child lacked detectable antibodies against both antigens. Of the 175 children with no history of whooping cough, 110 lacked detectable antibodies to both antigens, 23 had antibodies to both, 2 to toxin alone, and 40 to filamentous hemagglutinin alone. The data indicate that parental information on a previous history of whooping cough in their nonimmunized child is reliable, and that many infections with B. pertussis are subclinical or atypical. Exposure to other Bordetella species than B. pertussis, which is the only toxin-producing species, might be important for the development of FHA antibodies. A follow-up 2 to 4 years after the collection of serum samples of children without a history of whooping cough but with antibodies to one or both antigens indicated that serum antibodies to toxin, but not to filamentous hemagglutinin, may be protective against disease.