The procedure used by the hematologist who is treating a patient with aplastic anemia is as follows: bone marrow examination to confirm aplastic anemia; thorough physical examination plus chromosomal studies as indicated to rule out Fanconi's anemia; and investigation into possible etiologic factors for the aplasia. If severe (usually acquired) aplasia is diagnosed, family members should be HLA-typed immediately. The patient should be observed for approximately 2 weeks during the work-up, at bed rest and with platelet and RBC transfusion support as needed (but not from family members, in order to avoid sensitization in case a marrow transplantation will be done). If a match is found, the transplantation should be performed as quickly as possible. If there is no match, blood product support from random as well as family donors may be increased. Antilymphocyte serum should be given, and an immediate or delayed response awaited. Androgen therapy should be reserved as a last resort because it is of questionable efficacy except in the treatment of Fanconi's anemia. Putative causal environmental factors should be eliminated, and genetic factors must be considered. considered. In fact, all "acquired" aplastic anemia may occur on the background of homozygosity or heterozygosity for genes predisposing to aplastic anemia.