We administered cyclosporine to patients with aplastic anemia and pure red-cell aplasia as a multicenter clinical trial. The cyclosporine was given by a dose of 6 mg/kg orally daily for 16 weeks. The efficacy of cyclosporin for aplastic anemia was 13.8% after 8 weeks, and 28.0% after 16 weeks. The severity of the disease was improved, and the amount of blood transfusion was reduced. The efficacy of cyclosporine for patients with pure red-cell aplasia was 60.0% at 16 weeks treatment. Adverse effects of cyclosporine were observed in some patients and the hirsutism was seen most commonly. In laboratory data, an increase of serum creatinine was seen but was transient. Our results showed that cyclosporine is an effective drug for treatment of aplastic anemia and pure red-cell aplasia.