Throughout this century infants and young children have remained most susceptible to pertussis-related morbidity and mortality. In recent years infants younger than 6 months who are not old enough to have received three doses of the diphtheria-tetanus-pertussis vaccine and under-vaccinated preschool children have been at highest risk for pertussis-associated complications. Pertussis infection rates dropped dramatically after the whole cell pertussis vaccine came into widespread use, and an all-time low in reported cases in the United States was reached in 1976. Just as widespread immunization helped control the incidence of pertussis, it has probably been the primary factor in reducing pertussis-related mortality. Despite a stable or increasing vaccination rate in the United States since 1962, pertussis infection rates have been rising since the early 1980s. In 1993 the number of cases of pertussis reported represented an 82% increase over reported cases during the previous year and the highest incidence of pertussis since 1967. In 1993 pertussis became the most commonly reported vaccine-preventable disease among children in the United States younger than 5 years old. Growth of a susceptible adult population appears to be the primary factor contributing to the resurgence of pertussis in the United States; widespread immunization has reduced the potential for individuals to acquire exposure-induced immunity. It has been suggested that the majority of patients now infected with Bordetella pertussis are adults. Several studies have confirmed the importance of pertussis as the cause of persistent cough among teenagers and adults. As the diagnosis of pertussis goes unrecognized in these older patients and treatment is delayed or administered only partially, adolescents and adults have become an important source for transmission of B. pertussis infection to other household members, particularly infants and young children who are not adequately immunized.