A 4-month-old girl diagnosed with familial hemophagocytic lymphohistiocytosis underwent a matched unrelated, umbilical cord blood transplant. Six weeks later she developed severe acute autoimmune hemolytic anemia and thrombocytopenia requiring multiple transfusions. This was refractory to high-dose steroid and intravenous immunoglobulin, but did respond to Rituximab (anti-CD20 monoclonal antibody) 375 mg/m. Hemolysis recurred after steroid tapering but responded to a second course of Rituximab. This case report highlights the difficulty in managing posttransplant autoimmune hemolytic anemia. © 2007 Lippincott Williams & Wilkins, Inc.