During follow-up after allogeneic stem cell transplantation (SCT), patients frequently lose their immunity to infectious agents such as measles. The aim of this study was to analyze the influence of different factors on measles immunity. In total, 395 patients with a disease-free survival of at least 1 year were included. Measles vaccination was given at 2 years after SCT to children and young adults without chronic GVHD or ongoing immunosuppression. In all, 264 patients had matched sibling donors and 131 either mismatched family or unrelated donors. Totally, 318 patients received bone marrow and 77 peripheral blood stem cells. Overall, 375 patients had undergone myeloablative and 20 nonmyeloablative conditioning. Out of 395 patients, 133 (34%) were seronegative to measles. In multivariate models, younger age or being vaccinated to measles, rather than previous measles disease, before transplantation were risk factors both for becoming seronegative and to have doubtfully protective immunity to measles. Acute GVHD grade II-IV was a risk factor for seronegativity and blood stem cells a risk factor for doubtfully protective immunity. Children and young adults previously immunized to measles have a high risk for becoming vulnerable to a measles infection. Since measles is again circulating in many countries and measles is a serious infection after SCT, vaccination should be considered.