Background.Antibody persistence was studied in 5.5-year-old Swedish children who in infancy completed a vaccine trial of a combined diphtheria toxoid, tetanus toxoid, acellular pertussis, inactivated polio andHaemophilus influenzaetype b conjugate vaccine. Three priming doses at ages 2-4-6 months induced higher geometric mean concentrations of antibodies for all antigens than did two doses at 3-5 months, but there were no differences in proportions with protective antibody concentrations. After the booster dose administered at 13 or 12 months of age, respectively, there were no differences in concentrations or proportions between the groups.Methods.In the present follow-up serum samples from 180 of the 228 vaccinees, 88 from the 4-dose and 92 from the 3-dose group, were 4.5 years later again tested for antibodies.Results.The two groups did not differ significantly in antibody concentrations or proportions with antibodies above protective or other defined levels, with the exception of poliovirus type 3 (P≤ 0.01). In all 89% had ≥0.01 IU/ml antibodies against diphtheria by enzyme-linked immunosorbent assay and 76% by the Vero cell neutralization test, 93% had ≥0.01 IU/ml antibodies against tetanus, 96 to 99% had detectable antibodies against the polioviruses and 97% had ≥0.15 &mgr;g/mlH. influenzaetype b antibodies. As for pertussis only 44% had detectable antibodies against pertussis toxoid by enzyme-linked immunosorbent assay but 99% by Chinese hamster ovary cell neutralization test, and 94% had detectable antibodies against filamentous hemagglutinin.Conclusion.We found the persistence of antibodies satisfactory, with no clinically relevant differences in antibody concentrations demonstrated between children vaccinated according to a three dose or a four dose schedule in infancy.