We investigated efficacy and toxicity of lenalidomide in 24 heavily pretreated myeloma patients with a median age of 59 years (range: 37-70) and relapse after allo-SCT. Lenalidomide was given at a dose of 15 mg (n ¼ 4), or 25 mg (n ¼ 20), orally once daily on day 1 to day 1 every 28 days, with (n ¼ 20) or without (n ¼ 4) DHAP. The median number of lenalidomide cycles was five (range: 2-17). Major side effects were leukopenia (grade 4: 4%, grade 3: 21% and grade 2: 17%) and thrombocytopenia (grade 3: 17% and grade 2: 29%); infectious complications were observed in 50%. Non-hematological toxicity consisted of muscle cramps (n ¼ 9), fatigue (n ¼ 5) and constipation (n ¼ 2). Mild grade I-II GVHD was seen in three patients. Response was achieved in 66%: CR in 8%, VGPR in 8%, PR in 50% and SD in 13%. The median time to progression was 9.7 months (95% confidence interval (CI): 7.5-11.9), and median OS was 19.9 months (95% CI: 17.3-22.5). Immunomonitoring after lenalidomide showed significant increase of activated NK (NKp44 þ ) and T (HLA-DR þ ) cells, as well as regulatory T cells (CD4 þ , CD25 þ , CD127 lo ), supporting an immunomodulating anti-myeloma effect of lenalidomide.