Nine adult patients with chronic idiopathic thrombocytopenic purpura (ITP) were treated with cyclosporin. Their platelet counts were all below 5 x 10(4)/microliters. It was administered orally at 5 mg/kg/day for 8 weeks. In one patient, the platelet count increased over 10 x 10(4)/microliters in 4 patients it did over 5 x 10(4)/microliters. Gingival hyperplasia was observed in one patient. Renal dysfunction was not observed in any patients. The elevation of PAIgG declined during the period of treatment. These results suggest that this therapy may be useful in refractory idiopathic thrombocytopenic purpura.