Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired genetic disorder of the bone marrow that produces intravascular hemolysis, proclivity to venous thrombosis, and hematopoietic failure. Mutation in the PIG-A gene in a stem cell aborts synthesis of glycosyl-phosphoinositol (GPI) anchors and therefore expression of all GPI-anchored proteins on the surface of progeny erythrocytes, leukocytes, and platelets. The hemolytic anemia of PNH is well understood, and erythrocyte susceptibility to complement may be treated with anti-C5a monoclonal antibody. The pathophysiology of PNH cell clonal expansion and its association with immune-mediated marrow failure are not understood, but PNH/aplasia responds to immunosuppressive regimens such as antithymocyte globulin and cyclosporine. The mechanism of thrombosis in PNH is also obscure, but frequently fatal clotting episodes may be prevented by Coumadin (Bristol-Myers Squibb Pharma Co., Wilmington, DE) prophylaxis.