As aluminium in vaccines has been associated with the incidence of local side effects occurring after vaccination, this observer-blind randomised clinical trial was designed to evaluate the effect of lowering the aluminium content of a combined reduced-antigen-content dTpa vaccine on immunogenicity and safety when administered to healthy adolescents aged 10-18 years. A total of 647 subjects were enrolled, 224 (35%) received a dTpa formulation with 0.5 mg aluminium, 209 (32%) a formulation with 0.3 mg aluminium and 214 (33%) a formulation with 0.133 mg aluminium. One month after boostering, all subjects were seroprotected against diphtheria and tetanus toxoids. All subjects were seropositive for anti-FHA and anti-PRN but 4% of the initially seronegatives in both reduced aluminium groups did not seroconvert for anti-PT. Booster responses did not differ significantly between groups for any antibody, but post booster vaccination anti-PT GMC's differed significantly between groups and decreased when vaccine aluminium content decreased. No clear difference between study groups in local or general side effects was demonstrated. The most frequently reported symptoms after vaccination were injection site pain (89.5-90.7%), fatigue (42.1-47.4%) and headache (41.1-45.1%). This study showed that the aluminium content has a specific influence on the immunogenicity of this dTpa vaccine.