A subanalysis of a recent cohort efficacy trial of a pertussis vaccine was performed to determine its efficacy against cough illnesses due to Bordetella parapertussis infections. Infants received four doses of either the Lederle/Takeda acellular pertussis component diphtheria and tetanus toxoids and pertussis (DTaP) vaccine or the Lederle whole-cell component diphtheria and tetanus toxoids and pertussis (DTP) vaccine at 3, 4.5, 6, and 15-18 months of age; controls received three doses of diphtheria and tetanus toxoids (DT) vaccine only. All subjects were prospectively followed for cough illnesses of > or = 7 days' duration; cases of B. parapertussis infection were confirmed by positive culture, household contact, or serology. Seventy-six cough illnesses due to B. parapertussis were identified; 24 occurred in 929 DTaP recipients, 37 in 937 DTP recipients, and 15 in 321 DT recipients, resulting in an efficacy of 50% for DTaP vaccine (95% CI [confidence interval], 5% to 74%) and 21% for DTP vaccine (95% CI, -45% to 56%). The data in the present analysis suggest that the Lederle/Takeda DTaP vaccine but not the Lederle whole-cell component DTP vaccine has efficacy against B. parapertussis infection.