Although allogeneic stem cell transplantation (allo-SCT) is a potentially curative treatment option for multiple myeloma (MM), it is not recognized as a standard of care because of the high associated incidences of both treatment related mortality and relapse. We administered lenalidomide (Len) as maintenance therapy for patients with MM undergoing allo-SCT who were at high risk of disease relapse. Graft-versus-host disease was induced by Len administration in two patients, but was manageable with dose reduction. Although Len has a direct anti-myeloma effect and can also induce tumor immunity against residual myeloma cells, it is important to identify how to optimize the safety and the effects of Len administration after allo-SCT. Further accumulation of data including those from prospective clinical trials is urgently needed.