Paroxystic nocturnal hemoglobinuria (PNH) is an acquired hemolytic anaemia related to an increase susceptibility of erythrocytes to complement-mediated lysis. PNH is a clonal disease of an hematopoietic stem cell which lost, by mutation, the ability to synthesized phospholipid anchor of membranous proteins, i.e. complement regulatory proteins: DAF, C8BP or CD59. The clinical features of PNH are hemoglobinuria episodes associated with chronic hemolytic anaemia or pancytopenia with active bone marrow or aplastic anaemia. The clinical course is marked by severe thrombotic complications (such as Budd-Chiari syndrome), hemorrhages or infections. The diagnosis is confirmed by in vitro hemolysis tests, and now by facs analysis of cell membrane expression of deficient proteins. Different treatments have been proposed with various results (corticosteroid therapy, androgens, chemotherapy...) but the only way to eliminate the abnormal clone appears to be related bone marrow allograft.