A large single blind, multi-centre study involving 1779 children was performed in Italy. Infants, aged between 12 and 27 months were divided between two groups: group A received a single dose of a new MMR vaccine, 'Priorix'(3), while group B received a widely used MMR vaccine, Triviraten(4). Solicited local and general symptoms were recorded using diary cards and antibody levels were measured, prior to and 60 days post-vaccination, using ELISA assays. The incidence of solicited symptoms (evaluated in 1754 subjects) was comparable between groups, with the exception of fever which was significantly lower in group B. Immunogenicity was evaluated in 686 subjects. Of note, was the significantly higher anti-mumps seroconversion rate (p<0.001) observed in group A (97.0%) compared to group B (35.4%). However the anti-measles and anti-rubella seroconversion rates were equivalent between groups. Significantly higher (p<0.001) post-vaccination GMTs were in group A vs group B for anti-measles (2830 vs 784 IU/ml) and anti-mumps (1640 vs 469 U/ml), however the anti-rubella GMTs were significantly higher (p<0.001) in group B (117.6 IU/ml) compared to group A (92.6 IU/ml). The persistence of antibodies in 35 subjects was assessed 1 year after vaccination and the results showed no appreciable decline in titres with either vaccine. The trial demonstrates 'Priorix' is well tolerated and highly immunogenic.