Bone marrow transplant (BMT) recipients are routinely reimmmunized with the childhood vaccine series after transplantation excluding the live attenuated vaccines. In this study, the clinical and serologic responses to measles, mumps and rubella (MMR) vaccine in children after BMT was assessed. Twenty-two BMT recipients were vaccinated with MMR II (MSD). All were at least 2 years post-BMT and without GVHD. Their underlying conditions were leukemia (11), aplastic or Fanconi's anemia (7), thalassemia (3) and metabolic disease (1). All were allogeneic transplants with matched related donors. The mean age at transplantation was 6.9 years. There were no reported adverse effects of the vaccination. Antibody status for MMR was determined using commercial assays (IFA and ELISA) on paired specimens. The mean interval between transplantation and vaccination was 48 months. Pre-vaccination, no BMT recipient was sero-positive for all three, but 23% were positive for measles, 31% for mumps and 14% for rubella. Post-vaccination, 68% of BMT recipients were sero-positive for all three, with 77% for measles, 87% for mumps and 91% for rubella. Therefore, MMR vaccination at 2 years or later after BMT in paediatric recipients without GVHD was safe and significantly increased the proportion sero-positive for MMR.