15 patients with severe aplastic anemia were treated with an immunosuppresive regimen consisting of Cyclosporin-A(CsA) and highdose méthylprednisolone (HMP). Initially CsA was given orally at a dosage of 10 mg/kg/day and monitoring the serum level of the drug, blood levels of 200-500 ng/ml were maintained thereafter. CsA treatment was continued at least 3 months at which time the treatment was terminated in unresponsive patients depending on the peripheral cell counts. HMP was administered simultaneously with CsA at 30 mg/kg/day IV for 3 days, 20 mg/kg/day IV for 3 days and 10 mg/kg/day IV for 4 days. Within 3 months improvement of hematopoiesis was seen in 6 patients (40 %). First signs of response were seen in 21-69 (34) days. This was followed by complete remission in 3 patients, partial remission in 2 patients and minimal improvement in 1 patient. Remissions were proved to be dependent on the continued administration of CsA. Sequential changes of T-cell subsets using APAAP method revealed an increasing ratio of T-helper cells to T-suppressor cells. This Observation strongly suggests the role of T-cells in the pathogenesis of some cases of aplastic anemia.