Selected patients with Myelodysplastic Syndromes (MDS) are responsive to immunosuppressive therapy, suggesting that hematopoietic suppressive T cells have a pathogenic role in ineffective hematopoiesis. We assessed T-cell receptor (TCR) clonality through combined flow cytometry and molecular analysis of the complementarity determining region (CDR)-3 of the T-cell receptor-Vb gene. We identified clonal T cells in 50% of MDS patients (n ¼ 52) compared to 5% of age-matched normal controls (n ¼ 20). The presence of T-cell clones was not associated with features linked previously to immunosuppression response, including WHO diagnostic category, karyotype, marrow cellularity, IPSS category, sex or age p60. Using flow cytometry to identify expanded Vb-families, we found that T cells showed greater expansion in the bone marrow compared with peripheral blood, and were characterized as CD8 þ /CD57 þ /CD28 À effector T cells. Expanded effector T cell were CD62L negative and expressed the natural killer C-lectinfamily receptor NKG2D and CD244 (2B4). We conclude that clonal T-cell expansion is common among all MDS prognostic subgroups.