Paroxysmal nocturnal hemoglobinuria is an acquired clonal disorder of the hematopoietic stem cell in which intravascular hemolysis is due to an intrinsic defect in the membrane of red cells that makes them increasingly susceptible to lysis by complement. The phenotypic hallmark of PNH cells is an absence or marked deficiency of GPInchored proteins such as CD 59 + , CD 55 + and others which normally protect cells from the action of complement. PHN is closely associated with aplastic anemia. Some degree of bone marrow failure is always present. Management of PNH is complicated by a highly variable clinical picture and course. Some patients have severe anemia aggravated by hemolytic crises and associated thromboses. Bone marrow failure is accompanied with frequent infections and hemorrhagic manifestations due to thrombocytopenia. With the exception of marrow transplantation, no definite therapy is available. In the exceptional circumstance in which the patient has a syngeneic twin, bone marrow transplantation is the most appropriate therapy for severe PNH because of absence of graft-versus-host disease. In general syngeneic transplantation without preconditioning has been unsuccessful because abnormal hematopoiesis returns. Allogeneic bone marrow transplantation has been used, but the transplantassociated morbidity and mortality are high due mainly to the fatal graft-versus-host disease and severe posttransplant marrow failure. Use of an unrelated donor transplant has to be considered as contraindicated. PNH is associated with striking predisposition to intravascular thrombosis which ofter involves the portal system or the brain. Fatal thromboses account for about 40-50 % of all deaths in patients with PNH. The etiology of the thrombophilia in PNH is not fully clarified. Anticoagulation or thrombolytic therapy is required for treatment of venous thrombosis, the lather being recommended for life-threatening thromboses involving hepatic veins or inferior vena cava. Prophylactic anticoagulation in patients without contraindications such as severe thrombocytopenia seems to be justified. However, whether such therapy may be efficacious in reducing the incidence of thromboses or affect survival is conjectural. PNH patients have varying degree of platelet activation and some authors suggest that antiplatelet therapy might be efficacious in reducing the incidence and severity of venous thrombosis in PNH. Pregnancy is hazardous. Female patients should avoid the use of oral contraceptives. Pregnant patients require combined care of an experienced hematologist and obstetrician specialized in the management of highisk pregnancies.

        Key words: Paroxysmal nocturnal hemoglobinuria - Bone marrow transplantation - Thromboembolic complications – Prophylaxis - Therapy - Pregnancy