In order to evaluate the optimum number of doses for catch-up primary vaccination against pertussis, 248 Swedish children 3-4 years of age were randomized to receive either two or three doses of a three-component or a five-component acellular pertussis vaccine. Adverse reactions were mild, but increased with increasing number of doses, especially in a subgroup of 17 children with serological signs of earlier pertussis. There were no clinically significant differences between the two vaccines. Antibody levels against pertussis were higher after two doses in this age group than after three doses in infancy. A primary vaccination program for older children can use two instead of three doses.