Optimal salvage treatment for multiple myeloma relapsing after allogeneic stem cell transplantation remains to be determined. Usually, such patients have been heavily pre-treated and present at relapse with a relatively refractory disease. Immunomodulatory properties of lenalidomide may be beneficial by facilitating a graft-versus-myeloma effect after allogeneic stem cell transplantation. However, the safety of such treatment is still under debate. We conducted a multicenter retrospective study and included 52 myeloma patients receiving lenalidomide alone or in combination with dexamethasone as salvage therapy after allogeneic stem cell transplantation. The first aim was to assess the efficacy and tolerance of this drug. The second aim was to evaluate its potential immunomodulatory effects evaluated on the occurrence of acute graft-versus-host disease under treatment. In this cohort, we show that lenalidomide can induce a high response rate of 83% (including 29% complete response). On lenalidomide therapy, 16 patients (31%) developed or exacerbated an acute graft-versus-host disease, which was the only factor significantly associated with an improved anti-myeloma response. Side effects were mostly reversible, whereas 2 deaths (4%) could be attributed to treatment toxicity and to graft-versus-host disease, respectively. With a median follow up of 16.3 months, the median overall and progression free survival were 30.5 and 18 months, respectively, independently of the occurrence of acute graft-versus-host disease under lenalidomide. Lenalidomide can induce high response rates in myeloma relapsing after allogeneic stem cell transplantation at least in part by triggering an allogeneic anti-myeloma response. Induced graft-versus-host disease has to be balanced against the potential benefit in terms of disease control. Further immunological studies would help us understand lenalidomide immunomodulatory activity in vivo.