Aplastic anemia is a disease of the bone marrow. Bone marrow is the material in the center of bones that produces blood cells. Stem cells are the cells in the bone marrow that become red blood cells (cells that carry oxygen), white blood cells (cells that fight infection), and platelets (cells that help blood to clot). In aplastic anemia, the bone marrow does not produce stem cells, resulting in abnormally low numbers of all blood cells. Aplastic anemia is difficult to treat and can be fatal. The best hope for curing the disease has been treatment with the drug cyclophosphamide to destroy the abnormal bone marrow, followed by replacement with normal bone marrow (bone marrow transplantation). Unfortunately, many patients do not have appropriate bone marrow donors, and bone marrow transplantation has many serious complications. An earlier report suggested that some people with aplastic anemia can be free of the disease for long periods after treatment with only high-dose cyclophosphamide and do not require bone marrow transplantation.