A 38-year-old male patient with chronic myelocytic leukemia in the first chronic phase underwent bone marrow transplantation (BMT) from an HLA identical sibling. He developed chronic graft-versus-host disease and his condition gradually deteriorated. Fourteen months after BMT, acute progressive anemia, thrombocytopenia, reticulocytosis, increased serum lactic dehydrogenase and increased serum bilirubin were revealed following treatment with cyclosporine A (240 mg/day i.v.), prednisolone (60 mg/day i.v.) and azathioprine (100 mg/day p.o.). Red blood cell fragmentations were also found microscopically. At that time, the serum cyclosporine A trough level was 1,300 ng/ml by the polyclonal antibody RIA method. These symptoms were resolved by discontinuation of cyclosporine A and administrations of aspirin, cilostazol, and dipyridamole as anti-platelet agents. We consider this phenomenon to be micro-angiopathic hemolytic anemia due to a serum high cyclosporine A level which resulted from the concomitant use of cyclosporine A with prednisolone.