The study was aimed to detect the expressions of costimulatory molecules on CD3(+)CD4(+) T cells so as to accumulate informations for investigation of mechanism of myelodysplastic syndrome. 11 healthy blood donors as control and 38 patients with MDS de novo were studied. 38 MDS patients were divided into RA/RARS group and RAEB/RAEB-t group according to FAB classification. The expressions of CD28, CD154, CTLA-4, PD-1, CD25 on CD3(+)CD4(+) T cells in peripheral blood were detected by FCM. The results indicated that as compared with normal controls, the expression of CD28 in MDS patients decreased, and CD154 increased. The percentages of CTLA-4, PD-1 and CD25 in MDS were obviously higher than that in normal controls; the differences of CTLA-4, PD-1 and the ratio of CTLA-4/CD28 between RAEB/RAEB-t and RA/RARS were more significant with progressing of MDS. In conclusion, the expressions of costimulatory molecule in MDS patients were abnormal, which may be involved in the pathogenesis of MDS.