BACKGROUND: A study comparing diphtheria immunity in Norwegian and Russian schoolchildren indicated low immunity against diphtheria in Norwegian children before the booster dose given at age 11 years. The pertussis epidemic in Norway 1997-98 demonstrated decreasing vaccine immunity from age 5-6 years. The possibility of improving immunity against both diseases by a booster dose during early school age is therefore under consideration. MATERIAL AND METHODS: Immune response and adverse events were studied after a combined vaccine against diphtheria, tetanus, pertussis (acellular) and poliomyelitis (DTPa-IPV) given at seven years of age, and a combined vaccine against diphtheria, tetanus and pertussis (acellular) (DTPa) at 11 years of age, in two parallel trials including 124 and 83 participants respectively. RESULTS: The trials confirmed that the diphtheria immunity is lower than it ideally should be in more than 40% of children before the booster dose at age 11. Pertussis immunity is difficult to assess because there is no clear relationship between antibody levels and protection. All study participants responded well to all vaccine components. The 11-year-old children reported higher occurrence of adverse events than the 7-year-olds. All adverse events were brief and none were serious. INTERPRETATION: The results indicate that a booster dose of DTPa-IPV in early school age would give better protection against diphtheria and pertussis in Norwegian schoolchildren, without unacceptable side effects.