Myelodysplastic syndrome (MDS) and T-cell large granular lymphocytic disease (T-LGL) are bone marrow failure disorders. Successful use of immunosuppressive agents to treat cytopenia in MDS and LGL suggests a common pathophysiology for the two conditions. Of 100 patients with initial diagnoses of either MDS or T-LGL referred to the National Institutes of Health for immunosuppressive treatment of cytopenia, nine had characteristics of both T-LGL and MDS (T-LGL/MDS). Fifteen patients with T-LGL received cyclosporin (CSA) (10 responses). Eight out of nine patients with T-LGL/MDS received CSA (two responses) and one patient received ATG (one response). Of 76 patients with MDS, eight received CSA (one response) and 68 received ATG (21 responses). The response to immunosuppression was significantly lower in patients with T-LGL/MDS and MDS than in patients with T-LGL disease alone (28% vs. 66%, P = 0.01). The proportion of T-helper cells and T-suppressor cells with an activated phenotype (HLA-DR(+)) was increased in patients with T-LGL, T-LGL/MDS and MDS, but the increase in activated T-suppressor cells in patients with T-LGL/MDS was not statistically significant. Autoreactive T cells may suppress haematopoiesis and contribute to the cytopenia in T-LGL and some patients with MDS, leading to T-LGL/MDS. The lower response rate of MDS or T-LGL/MDS to immunosuppression, compared with T-LGL alone, may reflect the older age and intrinsic stem cell abnormalities in MDS and T-LGL/MDS patients.