A placebo-controlled study of varicella vaccine given either with or 6 weeks after measles-mumps-rubella (MMR) vaccine was undertaken in healthy children (mean age 16 months). A total of 101 varicella-zoster virus antibody-negative children completed the study. Serologic response to MMR vaccine was excellent (nearly 100%) and not significantly affected by the administration of varicella vaccine. Seroconversion in response to varicella vaccine was excellent and was not affected by MMR vaccine. No significant differences in fever or skin rashes between those receiving MMR vaccine with varicella vaccine or MMR vaccine with placebo were noted, but fever and skin rashes were more frequent after the first immunization (MMR with varicella vaccine or MMR vaccine with placebo) compared with the second (varicella vaccine or placebo injection). Symptoms of fatigue, irritability, and upper respiratory tract infections were more common after MMR vaccine was given regardless of whether it was given simultaneously with varicella vaccine or placebo injection.