OBJECTIVES: This study describes the role of adults in 57 household outbreaks of pertussis. METHODS: Parents/guardians of children with suspected pertussis seen at a university hospital were interviewed. Once a case of pertussis was confirmed, all household members were enrolled in the study. The US Centers for Disease Control and Prevention (CDC) definitions for pertussis were used . RESULTS: Among 349 household members, 150 were adults and one in five had pertussis during the household outbreaks. Of the adult cases, 70.6% were aged 19-39 years. The secondary attack rate in adults was 12.6%. Adults were the primary case in a 21.1% of households, resulting in 43% of all secondary cases. Most adult cases had typical symptoms of pertussis, but none had been diagnosed before the investigation. CONCLUSIONS: Adults, particularly those aged between 19 and 39 years, play an important role in pertussis transmission in households. Pertussis vaccination in adolescents/young adults may decrease the dissemination of pertussis in households.