Abstract： Objective To investigate the effect of dysfunction of T lymphocytes on clonal haematogenesis in patients with myelodysplastic syndrome (MDS). Methods The cytogentics, the subsets of lymphocytes and their activation in 76 patients with MDS were analyzed. Results There were 36 patients with normal karyotype and 40 patients with abnormal karyotype. The incidence of abnormal karyotype were 52.6 %. There were 24 cases (60.0 %) with trisomy 8 (+8) in 40 cases of abnormal karyotype. The expression rates of CD+3 CD-19 cells, CD+3 CD-4 CD+8 cells and CD+3 HLA-DR+ cells in MDS were significantly increased, and CD-3 (CD16CD56)+ cells were significantly lower than that in control group. The expression rates of CD+3 (CD16CD56)+ cells in MDS with abnormal karyotype were significantly higher than that in control group. The expression rates of CD+3 CD+4 CD-8 cells in +8 MDS were significantly lower than that in MDS patients with normal karyotype and with other abnormal karyotype. The ratio of CD4/CD8 in +8 MDS were significantly lower than that in control group. Conclusion The abnormalities of T cell subsets and functions in patients with MDS were observed and the proliferation of malignant clone was prevalent which indicated a poor prognosis in MDS with abnormal karyotype. Dysfunction of immunosurveillance was more aggravated in +8 MDS, which led to excess proliferation of malignant clone and over inhibition of remaining haematogenesis.