The serum IgG antibody response and decrease to 3 Bordetella pertussis antigens was compared in children with pertussis. Sera were obtained at the first clinical visit and 1, 3 and 12 months later from 89 children with > or = 3 weeks of paroxysmal cough. IgG antibodies to pertussis toxin (PT), to filamentous hemagglutinin (FHA) and to pertactin were determined with ELISA. Of 54 children with culture-confirmed pertussis or culture-confirmed familial exposure, 45 (83%) had a significant (> or = 3 fold) increase in PT IgG and 40 (74%) in FHA IgG antibodies, while only 29 (54%) had a significant increase in pertactin IgG antibodies. Significant decreases in PT, FHA and pertactin IgG antibodies were found in 34 (63%), 9 (17%) and 28 (52%) children, respectively. In the remaining 35 who did not have culture-confirmed disease, significant PT and/or FHA IgG antibody increases (criteria for pertussis according to the WHO definition) were found in 17 (49%). Only 6 of these 17 children had a significant pertactin IgG antibody increase. Of the remaining 18 children (who did not fulfil WHO criteria for pertussis), significant decreases in PT and/or FHA IgG antibodies were found in 13. We conclude that a serum IgG reaction to PT and FHA occurs in almost all children with pertussis. An increase in pertactin IgG antibodies occurs less frequently than against PT and FHA. Significant decreases in PT or FHA IgG antibodies in children with clinical pertussis might be of use as a diagnostic criterion in children brought late for examination.