Aplastic anemia is an acquired or constitutional trilineage bone marrow failure with peripheral pancytopenia, evidence of cellular hypoplasia of marrow, and replacement of normal cellular elements by fat. In some cases the marrow may be hypoplastic and the peripheral cytopenia relatively mild. The condition may persist for years and result in acute leukemia. In other cases, hypoplasia may involve only a single cell line. Basic research is directed toward understanding the multitude of cells, with a view toward selective manipulation of the cells themselves, interacting hormones, or cellular regulators. Early bone marrow transplantation presently offers the most hope when compatible donors are available.