An immunohistochemical study of T lymphocyte subsets on frozen substituted plastic embedding bone marrow sections obtained from 10 patients with myelodysplastic syndrome (MDS) was presented. The results of qualitative and quantitative immunohistochemical analysis are as follows: (1) Labile antigens of T lymphocytes were well preserved, thus allowing analysis of distribution of T lymphocyte subsets in situ: (2) the average number of T3, T4 and T8 lymphocyte of the diffuse infiltrate was about 2%, 0.4%, 0.5%, respectively, of all nucleated cells in bone marrow, and T4/T8 of T cells were below 1.0 in patients with MDS; (3) there were cases of RAS showing T lymphocyte aggregation in bone marrow, but no patient exhibited progressive refractory anemia with excess of blasts (RAEB) and RAEB in transformation (RAEBT). These findings indicated that the immunological abnormalities are of importance in the evaluation of pathogenesis and prognosis of MDS.