Measles, a highly contagious disease, is potentially serious in adult allogenic hematopoietic stem cell transplant (allo-HSCT) recipients. Because of the loss of immunity to vaccine preventable diseases after allo-HSCT, French Health Authorities (Haut Conseil de Santé Publique, HCSP) recommend (re)vaccination of all allo-HSCT recipients against measles-mumps-rubella (MMR) from 24 months post-transplant onwards, in the absence of graft-versus-host disease (GVHD) and at least 3 months after cessation of all immunosuppressive treatments, irrespective of measles serostatus. Nevertheless, some French experts argue that systematic assessment of measles antibody titre is justified after allo-HSCT, prior to revaccination, in order to avoid unnecessary revaccination of allo-HSCT recipients who are still seropositive. At the international level, recommendations also vary: the ECIL group and IDSA advocate revaccination of measles seronegative patients only, while some American Hematology experts recommend not to base the decision of revaccination on the serological status, given the inevitable loss of antibodies and specific long-term immune memory in the absence of revaccination. Several obstacles to the application of the recommendations can therefore be identified: (i) the risk of vaccine-transmitted disease due to the live-attenuated nature of MMR, (ii) the lack of robust data on the immunogenicity and tolerability of the MMR vaccine in this particular population, and (iii) conflicting recommendations to guide the decision of revaccination. This study aims at answering the question of whether some allo-HSCT recipients may retain a measles-specific cellular immune memory at distance from their allo-HSCT.