BACKGROUND: In the US, it is recommended that 4-6 year old children receive diphtheria-tetanus-acellular pertussis (DTaP), inactivated poliovirus (IPV), measles-mumps-rubella (MMR), varicella (V), and influenza vaccines. Data relating to the concomitant administration of combination DTaP-IPV vaccine (Kinrix; GlaxoSmithKline Biologicals) and influenza or V vaccines are currently limited. This study was undertaken to evaluate the immunogenicity and reactogenicity of Kinrix when co-administered with MMR (M-M-RII((R)), Merck & Co.) and Varivax (Merck & Co.) in 4-6 year old children. METHODS: Phase IIIb, open-label, non-inferiority study (NCT00871117). We randomized (1:1) healthy 4-6 year olds to receive Kinrix+MMR+V on day 0 (Group 1), or Kinrix+MMR on day 0, followed by V at month 1 (Group 2). We measured DTaP-IPV immunogenicity before and 1 month post-vaccination (prior to V vaccination in Group 2). We collected local and general solicited symptoms within 4 days after vaccination and serious adverse events (SAEs) through 6 months post-vaccination. RESULTS: We enrolled 478 subjects. One month post-vaccination, >95% of subjects in both groups had booster responses to diphtheria, tetanus and pertussis antigens and all subjects had seroprotective anti-poliovirus antibody titers. Immune responses in Group 1 were non-inferior to Group 2 for responses to DTaP-IPV antigens according to pre-specified criteria. Reporting of solicited local events at the DTaP-IPV site appeared to be similar between the two vaccine groups, as was reporting of solicited general adverse events within 4 days of vaccination; no vaccine related SAEs were reported. CONCLUSION: Concomitant administration of varicella vaccine with Kinrix and MMR did not impact the immunogenicity of diphtheria, tetanus, pertussis or poliovirus antigens. Both vaccine regimens were well tolerated. These results support the co-administration of DTaP-IPV, MMR, and V vaccines in 4-6-year-old children, providing protection against multiple diseases in a timely and efficient manner.