The rate of isolation of Bordetella parapertussis among children with cough during the follow-up of different clinical efficacy studies has been evaluated. In the Italian trial, a comparison of clinical characteristics between B. pertussis and B. parapertussis infections showed lower frequencies and shorter duration of typical symptoms of whooping cough such as paroxysmal coughing, whooping, and vomiting in the group of children affected with B. parapertussis infections. In about 70% of B. parapertussis infections, there was a two-fold increase of IgA or IgG anti-FHA from acute- and convalescent-phase serum specimens. The analysis of the distribution of B. parapertussis cases in children fully immunized with each pertussis vaccine suggested that vaccination is irrelevant in preventing B. parapertussis infection.