A case of a 64-year-old Japanese woman who developed aplastic anemia and idiopathic thrombocytopenic purpura with antibody to platelet glycoprotein IIb/IIIa 4 years following a resection of malignant thymoma is reported. Bone marrow was hypocellular and ferrokinetics revealed the prolongation of the half-time of plasma iron disappearance and a decrease in red cell utilization, findings compatible with a diagnosis of aplastic anemia. The life span of platelets was markedly decreased to 3.07 h, and the test for antiplatelet glycoprotein IIb/IIIa antibody was positive. The patient's serum had a suppressive effect on the formation of colonies of burst-forming unit-erythroid and colony-forming unit-granulocyte using normal bone marrow cells. These results suggest that the aplastic anemia in this patient may have been induced by some suppressive activity in the serum. Splenectomy followed by an administration of cyclosporine effectively restored the peripheral blood count.