This study was undertaken to assess the co-administration of an experimental measles-mumps-rubella-varicella vaccine (MMRV, GlaxoSmithKline Biologicals) with a combined diphtheria-tetanus-acellular pertussis-hepatitis B-inactivated poliovirus-Haemophilus influenzae type b conjugate (DTPa-HBV-IPV/Hib) vaccine in healthy children. Healthy children aged 12-23 months (N = 451) were randomised to one of three parallel groups to receive one dose of MMRV vaccine co-administered with a booster dose of DTPa-HBV-IPV/Hib vaccine (co-administration group), or one dose of MMRV vaccine alone (MMRV group), or a booster dose of DTPa-HBV-IPV/Hib vaccine alone (DTPa-HBV-IPV/Hib group). No differences in seroconversion rates for measles (>95%), mumps (>80%), rubella (>99%) and varicella (>98%) were seen between the co-administration group and the MMRV group. No differences in geometric mean titres (GMTs) were observed between the two groups with the exception of anti-measles titres, which were observed to be higher in the MMRV group than in the co-administration group (4,419.2 vs. 3,441.8 mIU/ml respectively). Immune response to the booster dose of DTPa-HBV-IPV/Hib vaccine was observed to be similar in the co-administration group and the DTPa-HBV-IPV/Hib group. Co-administration of the MMRV vaccine with a booster dose of DTPa-HBV-IPV/Hib vaccine was well-tolerated and did not exacerbate the reactogenicity profile of either vaccine. In summary, GlaxoSmithKline Biologicals' experimental MMRV vaccine was immunogenic and well-tolerated when administered with a booster dose of DTPa-HBV-IPV/Hib vaccine during the second year of life. The ability to co-administer the MMRV vaccine at the same time as other routine childhood immunisation vaccines could increase compliance with varicella vaccination in countries where this vaccine is already recommended and may facilitate implementation of varicella vaccination elsewhere.