The measurement of IgG anti-pertussis toxin (IgG anti-PT) antibodies by ELISA is a frequently used method for studying the antibody responses after pertussis vaccination and after Bordetella pertussis infection. Such responses vary according to the different vaccines used as well as to the immunization and infection history of the participants. In the present study, the decay kinetics of the IgG anti-PT antibody response was determined for 71 Danish children and adults with bacteriologically confirmed B. pertussis infection and for 20 Danish adults booster-vaccinated with an acellular pertussis vaccine. For both groups, biphasic decay was seen, but the individual antibody responses varied greatly. No differences related to age were seen. Within each group, individual decay profiles showed parallel log-linear decay for the late part of the response. Antibody half-life was calculated for the late, slower part of the biphasic response curves for both groups (>5 months after diagnosis for individuals with confirmed infection; >3 months for vaccinated individuals). The median half-life for post-infection antibodies was 221 days [interquartile range (IQR) 159-314 days, 36 individuals], and the median half-life for post-vaccination antibodies was 508 days (IQR 428-616 days, 14 individuals). This difference was statistically significant (P<0.0001). Thus, in this setting, we found that the IgG anti-PT antibody decay after an infection with B. pertussis is more than twice as fast as the decay after booster vaccination with an acellular pertussis vaccine. Such knowledge of the IgG anti-PT decay kinetics is crucial for interpretation of serological data that will be used either for diagnosis or for epidemiological studies and surveillance of B. pertussis infections.