We reported a prednisolone effective case of T 8 lymphocytosis. The patient was 83 years old male, who had suffered from hypoplastic anemia for more than 5 years before he admitted to our hospital. His blood picture was peculiar because adding to granulocytopenia, CD8 (+) large granular lymphocytes were dominant among his lymphocytes. A few days after administration of bolus methylprednisolone, prominent reticulocytosis and rapid improvement of anemia occurred and the latter continued for more than 1 year. We performed in vitro study on the mechanism of his cytopenias. The result of Southern blot analysis showed that his blood lymphocytes had a unique rearranged pattern of T cell receptor beta chain gene. Mononuclear cells had an inhibitory effect on autologous and allogenic CFU-E colony formation of bone marrow cells. These facts suggest that colony proliferating CD8 (+) lymphocytes had an inhibitory effect on erythropoiesis in vivo, and prednisolone blocked this inhibitory pathway in this patient.