We reviewed 400 bacteriologically confirmed cases of pertussis in infants and children during the past 18 years. Several changes in the epidemiology have occurred in the most recent six-year period. The incidence of whooping cough in children has decreased by at least 50%, but the proportion of cases occurring in infants younger than 12 weeks of age has doubled to 30% of all cases. Formerly most young infants acquired their illness from siblings or other children, but in the recent period adults in the household were the most common source of infection to neonates and young infants. This observation plus the increasingly high level of immunization in preschool and school-aged children suggest that young adults with waning immunity and mild illness are a major reservoir for transmission of pertussis to infants too young to be immunized.