From May, 1990, a new schedule of immunisation against diphtheria, tetanus, and pertussis (at 2, 3, and 4 months) replaced the previous more widely spaced schedule. A report that children had lower concentrations of diphtheria and tetanus antibodies a month after an accelerated schedule led us to undertake a controlled study to assess antibody response and the persistence of antibodies a year after immunisation in children receiving vaccine according to widely spaced and accelerated schedules. Concentrations of antibodies to diphtheria and tetanus toxoids and to Bordetella pertussis filamentous haemagglutinin (FHA) were measured by solid-phase radioimmunoassay (SP-RIA). We studied 57 children who received accelerated immunisation at median ages of 11, 16, and 21 weeks and two control cohorts (total n=82) who received vaccine at median ages of 15, 21, and 45 weeks. 6-8 weeks after the third dose the accelerated-schedule group had lower (p<0·0001) geometric mean concentrations of antibody to tetanus (0·522 [95% Cl 0·383-0·710] vs 3·43 [2·45-4·81] IU/mL), diphtheria (0 266 [0 179-0 396] vs 2·39 [0·616-3·53] IU/mL), and FHA (0 044 [0 030-0·063] vs 0·270 [0 196-0·374] units/mL) than the longer-schedule group. 12 months after the third dose the differences between the groups had narrowed (tetanus 0·197 vs 0·341 IU/mL, p=0·29; diphtheria 0·100 vs 0·131 IU/mL, p=0·64; FHA 0 014 vs 0·016 units/mL, p=0·72). At that time all children had tetanus antibody concentrations above protective levels (0·01 IU/mL); only 2 of 31 in the accelerated-schedule group and 3 of 31 in the longer-schedule group had diphtheria antibody concentrations below the protective level. The use of an accelerated schedule of diphtheria, tetanus, and pertussis vaccination is unlikely to lead to an increase in the proportion of children unprotected against these diseases before the preschool booster.