We treated 14 patients with aplastic anemia (6 severe, 8 moderate) who did not respond to high dose methylprednisolone, antilymphocyte globulin, or anabolic steroid with cyclosporin (CyA) for more than 4 weeks. Four of them (29%) showed clinical improvement to transfusion-independence. The response rate of patients who received CyA more than 5 weeks was 50% (4 out of 8). The dose of CyA given to the 4 patients who improved after the therapy was 5 to 9 mg/kg/day. One patient needed 11 weeks until the first sign of response (increase of reticulocytes) appeared. In the other three patients, platelets increased first in response to CyA within 5 weeks. Side effects such as hypertricosis and gingival hyperplasia were frequently seen during the CyA therapy, but since they were not so severe or transient, almost all patients tolerated the therapy. These results indicate that CyA can be an efficacious drug for patients with refractory aplastic anemia. Long term treatment with a relatively low dose of CyA may be important for obtaining a high response rate.