Paroxysmal nocturnal hemoglobinuria (PNH) results from a deficiency in inhibitors of activated complement. This lack leads to complement mediated intravascular hemolysis, to activation of coagulation system with increased risk of thrombotic complications and to various degree of bone marrow failure. A molecular basis of PNH is a somatic mutation of PIGA gene causing a lack of glycosyl phosphatidyl inositol which binds many important antigens to cell surface membrane including inhibitors of activated complement CD59 and CD55 antigens. Presence of CD59 and CD55 deficient cells in peripheral blood detected by flow cytometry is essential for diagnosis of PNH. Standard treatment of PNH includes transfusions of leukocyte depleted red blood cells, anticoagulation prophylaxis and administration of folinic acid. PNH patients with predominant signs of bone marrow failure and profound cytopenia may by candidates for stem cell transplantation or combination immunosuppression. Patients with severe hemo-lysis and/or thrombotic complications are indicated for administration of eculizumab - a monoclonal antibody against C5 part of complement. Key words: diagnosis - eculizumab - immunosuppression - paroxysmal nocturnal hemoglobinuria - pathogenesis - transplantation - treatment.