The vaccination program for pertussis (whooping cough) in the United States consists of giving multiple doses of pertussis vaccine to young children. A demographic model with a steady-state age distribution is used as a basis for building an epidemiologic model for the transmission of pertussis. This age-structured model includes vaccination of infants and children for pertussis with waning of both infection-acquired and vaccine-induced immunity. Computer simulations of the mathematical model between 1940 and 2040 show the changes that took place during the implementation phase of the U.S. program and predict only minor future changes in the age distribution and incidence of pertussis if the vaccination program is maintained at the 1995 level. The sensitivities of these results to changes in demographic and epidemiologic parameters, vaccine efficacy, duration of protection, and levels of vaccination coverage are investigated.