During 1974, eight of 37 (22%) Bordetella organisms isolated from patients in Cincinnate were Bordetella parapertussis. This is in contrast to other experience in the United States where parapertussis has comprised less than5% of the Bordetella species isolated and suggest that B parapertussis infection may be more common in this country than generally recognized. The failure to appreciate the presence of this infection may result from the lack of cultures taken from children with mild disease and the failure todistinguish B parapertussis from B pertussis. Ccultures were obtained from family members of three of the children with B parapertussis, and B pertussis was isolated from members of two families, including the mother and sister of a child who died of pneumonia and encephalopathy. These cases suggest that patients with severe disease associated with B parapertussis should be carefully evaluated for the possibility of dual infection caused by b pertussis.