Humoral immunity to measles, rubella and mumps was studied in 63, 36 and 16 patients 1, 2 and 3 years, respectively after autologous bone marrow transplantation (ABMT). Serologic examination was performed using antibody-ELISA. One year after ABMT, 7/57 patients (12%) who were seropositive to measles before ABMT, became seronegative, 8/44 (18%) to rubella and 3/51 (6%) to mumps. Among patients who were retested at 2 and 3 years, three more patients became seronegative to measles, one to rubella, and three to mumps. Nine of 12 children who had previously been immunized against measles were seropositive before ABMT, 3/7 to rubella and 5/7 to mumps, respectively. After ABMT, 5/9 became seronegative to measles, none to rubella and 2/5 to mumps. Six seronegative children were immunized with a live trivalent attenuated measles, mumps, and rubella vaccine 1 to 2 years after ABMT. Two children seroconverted to measles, six to rubella, and four to mumps. No side effects were observed. Most adult patients who have had the diseases of measles, rubella, or mumps naturally remain seropositive, while children who have been immunized commonly lose their immunity after ABMT.