Our population-based study among HMO members under seven years of age in 1993-2000 showed that frequency of diphtheria and tetanus toxoid vaccine (DT) use declined significantly following the introduction of acellular pertussis-containing (DTaP) vaccine. We also observed changes in indications for DT following the transition to DTaP among children under two years of age; notably, a decline in the proportion of children receiving DT due to a reported prior vaccine reaction and an increase in the proportion of children receiving DT due to parental request and a history of pertussis.