In a double-blind, placebo-controlled efficacy trial of a monocomponent pertussis toxoid vaccine, 3450 infants were randomly assigned to vaccination with diphtheria-tetanus toxoids with or without pertussis toxoid at 3, 5, and 12 months of age. Study children and family members were investigated for possible pertussis with cultures, serology, and polymerase chain reaction. Efficacy was 71% after 3 dose when the World Health Organization case definition of pertussis (which includes paroxysmal cough for 21 days or longer) was used. We report the efficacy in the subgroup of children who were exposed to pertussis in the household. Among study children exposed to pertussis in the household from the day of the third vaccination, 20 of 99 (20%) recipients of diphtheria-tetanus-pertussis toxoids vaccine and 64 of 79 (81%) recipients of diphtheria-tetanus toxoids vaccine had pertussis fulfilling criteria of the World Health Organization. The vaccine efficacy was 75% (95% confidence intervals 64% to 84%). In children who had received only two doses at the time of household exposure, vaccine efficacy was 66% (95% confidence intervals 15% to 90%) based on 4 cases among 32 household-exposed recipients of diphtheria-tetanus-pertussis toxoids vaccine and 13 cases among 35 household-exposed recipients of diphtheria-tetanus toxoids vaccine. In conclusion, the pertussis toxoid vaccine provides protection against pertussis both after household and community exposure.