A randomized trial was conducted to assess the immunogenicity and reactogenicity of yellow fever vaccines (YFV) given either simultaneously in separate injections, or 30 days or more after a combined measles-mumps-rubella (MMR) vaccine. Volunteers were also randomized to YFV produced from 17DD and WHO-17D-213 substrains. The study group comprised 1769 healthy 12-month-old children brought to health care centers in Brasilia for routine vaccination. The reactogenicity was of the type and frequency expected for the vaccines and no severe adverse event was associated to either vaccine. Seroconversion and seropositivity 30 days or more after vaccination against yellow fever was similar across groups defined by YFV substrain. Subjects injected YFV and MMR simultaneously had lower seroconversion rates--90% for rubella, 70% for yellow fever and 61% for mumps--compared with those vaccinated 30 days apart--97% for rubella, 87% for yellow fever and 71% for mumps. Seroconversion rates for measles were higher than 98% in both comparison groups. Geometric mean titers for rubella and for yellow fever were approximately three times higher among those who got the vaccines 30 days apart. For measles and mumps antibodies GMTs were similar across groups. MMR's interference in immune response of YFV and YFV's interference in immune response of rubella and mumps components of MMR had never been reported before but are consistent with previous observations from other live vaccines. These results may affect the recommendations regarding primary vaccination with yellow fever vaccine and MMR.