The epidemiology of whooping cough in a vaccinated population was studied during an outbreak of paroxysmal cough in an elementary school with 258 pupils in Turku, Finland. Nasopharyngeal specimens for isolation of Bordetella pertussis and/or sera for ELISA detection of antipertussis immunoglobulin M, A and G antibodies were taken from 94% of children who were prospectively followed for two months. Bordetella pertussis was isolated in six patients, and 17 culture-positive cases with Bordetella parapertussis were identified. Patients with Bordetella pertussis or Bordetella parapertussis were found simultaneously in the same classrooms. Comparison of immunoglobulin M responses to Bordetella pertussis and Bordetella parapertussis was used for differential diagnosis of these two infections. Twenty-six cases with pertussis and 27 cases with parapertussis were diagnosed. The results of this prospective study suggest that Bordetella parapertussis is a more common etiologic agent of mild respiratory tract infection among vaccinated school-aged children than is generally recognised. The possibility that Bordetella pertussis was converted to Bordetella parapertussis during this outbreak is discussed.