Whooping cough can be caused by either Bordetella pertussis or Bordetella parapertussis. Although the two species share an almost complete DNA identity, Bordetella parapertussis does not produce pertussis toxin, which is thought to be the main virulence factor of Bordetella pertussis. In order to elucidate the role of pertussis toxin in causing the typical symptoms of whooping cough, clinical information from 33 patients with culture-positive Bordetella parapertussis infection was collected and compared to that from 331 patients with infection caused by Bordetella pertussis. Isolated strains of Bordetella parapertussis lacked pertussis toxin expression, as was demonstrated by negative tests for histamine sensitization. This was further substantiated in vivo by a significantly lower leukocyte count in the parapertussis group as compared to the pertussis group. Frequencies of typical symptoms of whooping cough, such as paroxysmal coughing, whooping and vomiting, were almost identical in the two groups. Nocturnal coughing and contact anamnesis were noted more often in the Bordetella pertussis group. Children in the parapertussis group were significantly more often vaccinated with whole-cell pertussis vaccine than children infected with Bordetella pertussis. The results indicate that pertussis toxin may not play a decisive role in causing the typical symptoms of whooping cough, such as paroxysmal coughing, whooping and vomiting.