Healthy 4-6 y old children from Italy and Sweden immunized with DTPa and inactivated or oral polio vaccines at 3, 5 and 11-12 months of age, received 1 dose of combined DTPa-IPV (n = 211) or DTPa + IPV as separate doses (n = 205) in a randomized trial. The pre-booster seroprotection rates were similar in each group and were above 60% against all antigens except diphtheria (31.3% and 37.0%) and PT (21.5% and 25.9%) in the DTPa-IPV and DTPa + IPV groups, respectively. At least 99.5% of subjects had seroprotective antibody levels against diphtheria, tetanus and polioviruses and > or = 96% showed a vaccine response to each pertussis antigen after vaccination. Post-booster antibody levels increased at least 51-fold for anti-diphtheria and anti-tetanus, at least 18-fold for anti-pertussis antibodies and at least 32-fold for antibodies against all 3 poliovirus types, compared to prior levels. DTPa-IPV was comparable to DTPa + IPV in terms of seroprotection rates and mean antibody levels against each vaccine antigen. Similar reactogenicity profiles were observed between groups including swelling > 50 mm [13% (9.1, 18.7) vs 17% (12.4, 23.4)] or involving an adjacent joint [0% (-,-) vs 1.5% (0.3, 4.3)] and were consistent with previous reports. The combined DTPa-IPV vaccine could be used to add DTP valences to the IPV vaccine currently given to children in Scandinavia and Italy at 4-6 y of age and reinforce protection against 4 diseases.