Myelodysplastic syndromes (MDS) are a heterogeneous group of bone marrow disorders characterized by ineffective hematopoiesis and a tendency to develop leukemia. In some patients, laboratory and clinical evidence supports a role for the immune system in the pathogenesis of early MDS. Many younger patients who respond to immunosuppressive therapy with drugs such as antithymocyte globulin and cyclosporine have clonal expansions of cytotoxic CD8(+) T cells that suppress normal hematopoiesis, as well as expansion of CD4(+) helper T-cell subsets that promote and sustain autoimmunity. Immunosuppressive therapy can produce hematologic responses in some patients and may improve survival and halt leukemic progression. In this report, we describe a 56-year-old woman who presented with fatigue and easy bruising, eventually became pancytopenic, and was diagnosed with MDS. After treatment with a clinical protocol using alemtuzumab, an anti-CD52 antibody, her blood cell counts returned to normal and she has remained in complete remission for more than 2 years of follow-up. In this article, we review the pathobiology of immune dysregulation in MDS and summarize the role of immunosuppressive therapy in MDS.