A 19-month-old girl with idiopathic severe aplastic anemia refractory to multi-agent immunosuppressive therapy developed severe platelet alloimmunization following several months of platelet transfusions. She became refractory to human leukocyte antigen (HLA)-matched platelet transfusions and experienced frequent episodes of bleeding. She was treated with intravenous vincristine administered weekly for three doses and showed marked improvement in both clinical and laboratory response to platelet transfusions. When vincristine was held for 3 weeks, she again became refractory to HLA-matched platelet transfusion. Reinstitution of vincristine resulted in cessation of clinical bleeding and improved response to platelet transfusion. The mechanism of response likely involves selective delivery of cytotoxic drug to macrophages. To our knowledge this is the first reported case of alloimmune thrombocytopenia responsive to vincristine.