Long-term immunity to measles, mumps, and rubella viruses was studied in 57 patients after allogeneic bone marrow transplantation. Among patients who were seropositive at the time of transplant, 51% had retained antibodies to measles, 42% had retained antibodies to mumps, and 76% had retained antibodies to rubella 2 y later. There was no difference in the ability to retain antibodies to these viruses between patients with and those without chronic graft-versus-host disease (GVHD). Twenty seronegative patients without active chronic GVHD or ongoing immunosuppressive treatment were vaccinated with a live attenuated trivalent vaccine against measles, mumps, and rubella. No early or late side effects were detected after the vaccinations. The percentages of patients who seroconverted after vaccination were 77%, 64%, and 75% for measles, mumps, and rubella, respectively. Vaccination of transplant recipients with a live attenuated vaccine against measles, mumps, and rubella is safe and usually effective 2 y after transplant if the patients do not have active chronic GVHD or ongoing immunosuppressive treatment at the time of vaccination.