Paroxysmal nocturnal hemoglobinuria (PNH) is a rare acquired disease 
characterized by a clone of blood cells lacking glycosyl phosphatidylinositol 
(GPI)-anchored proteins at the cell membrane. Deficiency of the GPI-anchored 
complement inhibitors CD55 and CD59 on erythrocytes leads to intravascular 
hemolysis upon complement activation. Apart from hemolysis, another prominent 
feature is a highly increased risk of thrombosis. Thrombosis in PNH results in 
high morbidity and mortality. Often, thrombosis occurs at unusual locations, 
with the Budd–Chiari syndrome being the most frequent manifestation. Primary 
prophylaxis with vitamin K antagonists reduces the risk but does not completely 
prevent thrombosis. Eculizumab, a mAb against complement factor C5, effectively 
reduces intravascular hemolysis and also thrombotic risk. Therefore, eculizumab 
treatment has dramatically improved the prognosis of PNH. The mechanism of 
thrombosis in PNH is still unknown, but the highly beneficial effect of 
eculizumab on thrombotic risk suggests a major role for complement activation. 
Additionally, a deficiency of GPI-anchored proteins involved in hemostasis may 
be implicated.