AbstractA prospective, randomised clinical trial was conducted to evaluate the efficacy of two live attenuated mumps‐measles vaccines, the Urabe Am 9‐Schwarz and the Jeryl Lynn‐Moraten vaccine, in 400 young children aged 9 months‐4.5 years (median 13.4 months). Antibody responses to both vaccine components were measured by the enzyme‐linked immunosorbent assay (ELISA); 96.9% of the seronegative children who received the Urabe Am 9‐Schwarz vaccine showed satisfactory mumps antibody responses compared to 90% of the Jeryl Lynn‐Moraten vaccine recipients (P<.01). Similar proportions of both groups, 98.5% and 96.8%, respectively, developed measles virus specific antibodies. Both vaccines were equally well tolerated and clinical