During the acellular pertussis vaccine trial in Sweden, 4 children who were randomly assigned to receive the vaccine died of suspected or confirmed bacterial infections compared to 1 expected. There were no deaths in the placebo arm. This raised concern about the role of pertussis immunization in the development of serious infections. Through linking computerized immunization records with an active surveillance system for serious bacterial infections in children, the authors studied a cohort of 64,591 children immunized through Tennessee county health clinics who had a total of 158 episodes of invasive bacterial infections after a diphtheria and tetanus toxoids and pertussis (DTP) immunization. There were 8 invasive bacterial infections that occurred within the first 7 days following DTP immunization, yielding an age-adjusted relative risk (95% confidence interval) of 1.0 (0.5 to 2.0), compared to the interval 29 or more days following immunization. There were 7 and 20 infections in the 8- through 14- and 15- through 28-day intervals following DTP immunization, giving relative risks of 0.8 (0.4 to 1.7) and 1.2 (0.7 to 1.9), respectively. These data provide reassurance that the use of DTP vaccine is not followed by a large increased risk of serious bacterial infections.