We studied the immune compartment in patients with myelodysplastic syndromes. We show increased surface expression of activation markers (HLA-DR(+), CD57(+), CD28(-), CD62L(-)) on T lymphocytes in blood and bone marrow (n=131). T cell activation was not restricted to any relevant clinical subgroup (FAB, IPSS, cytogenetics) and did not correlate with blood counts or need for treatment. In vitro clonogenic growth of marrow mononuclear cells (n=18) was not influenced by T cells expressing these markers. In addition, using X-chromosome inactivation analysis (n=12) we demonstrate clonal involvement of NK and B cells in half of these patients. We conclude that although activated T lymphocytes can be found in MDS, their role in disease pathogenesis remains unclear in the majority of patients.