Our present study seeks to illuminate roles of Th17 cells in the pathogenesis of MDS patients in different risk groups through measuring the frequency of Th17 cells, detecting levels of related cytokines in patients with different risks of MDS, analyzing the correlation between Th17 cells percentage and hematological parameters of MDS patients, and testing levels of cytotoxic molecules expressed in CTL before and after human recombinant IL-17 (rhIL-17) stimulation, ultimately to explore the role of Th17 cells in the tumorigenesis of MDS. The gene discussed is IL17A; the disease is myelodysplastic syndrome.