At the age of 7-8 years a booster of diphtheria, tetanus, acellular pertussis and polio vaccine is recommended for children in Norway. In this cross-sectional study we have analysed the antibody levels against pertussis vaccine antigens in sera from 498 children aged 6-12 years. The purposes of this study were to investigate the duration of the booster response against the pertussis vaccine antigens pertussis toxin (PT) and filamentous haemagglutinin (FHA); to determine the presence of high levels of pertussis antibodies in absence of recent vaccination; and to analyse how booster immunisation may interfere with the serological pertussis diagnostics. Prior to the booster the IgG antibody levels against PT revealed a geometric mean of 7.3IU/ml. After the booster the geometric mean peak anti-PT IgG response reached to 45.6IU/ml, followed by a steady decline in antibody levels over the next few years. The IgG anti-FHA levels followed the anti-PT IgG profiles. Three years after the booster the geometric mean IgG levels were only slightly above pre-booster levels. Prior to the booster 44% of the sera contained 5IU/ml of anti-PT IgG compared to18% 3 years after and 30% 4 years after the booster. When recently vaccinated children were excluded, 6.2% of the children had anti-PT IgG levels above 50IU/ml which may indicate pertussis infection within the last 2 years. This study indicates that the currently used acellular pertussis vaccines induce moderate immune responses to the pertussis antigens and that the antibodies wane within few years after the booster. This lack of sustained immune response may partly be responsible for the increased number of pertussis cases observed in this age group during the last years.