To measure the clinical effect of adding a whole cell pertussis component to diphtheria/tetanus vaccine (DT) given as a pre-school booster, 190 children aged 4-5 years were randomised by a double-blind method to receive either diphtheria/tetanus/pertussis (DTP) or DT vaccine in a 1:1 ratio at selected clinics in England. The geometric mean antibody titres to each of the three pertussis antigens were at least sixfold higher in the DTP than the DT vaccine group and equalled or exceeded those in infants immediately after primary immunisation with DTP vaccine. There were no significant differences between DTP and DT vaccinated children in their diphtheria and tetanus antitoxin levels. The frequency of large local reactions and systemic symptoms such as crying and a disturbed night was 2-3-fold higher in the DTP vaccinees than in the DT vaccinees. Medication was given to 44% of DTP and 23% of DT vaccinees (p = 0.006). Although the change to whole cell DTP vaccine at school entry would result in good pertussis antibody titres, the 2-3-fold increase in reactogenicity that would be caused may be unacceptable at a time when whooping cough is not circulating widely. Evaluation of acellular DTP vaccines given as a pre-school booster in children vaccinated under the accelerated schedule is planned.