The primary clinical manifestations of paroxysmal nocturnal hemoglobinuria (PNH) are hemolytic anemia, marrow failure, and thrombophilia. However, PNH is not a simple binary diagnosis and both flow cytometric characterization of glycosyl phosphatidylinositol–anchored protein expression on peripheral blood cells and marrow analysis are required for comprehensive disease classification. For optimum management, the contribution of both hemolysis and marrow failure to the complex anemia of PNH should be determined. Complement inhibition by eculizumab is a promising new approach to treating the hemolytic anemia. Stem cell transplantation is potentially curative, but the decision on use is best made on a case-by-case basis because of the heterogeneous natural history of the disease. PNH clone size and ethnic/geographic factors appear to influence thrombophilic propensity, but a consensus on prophylactic anticoagulation has not been reached. Involvement of unusual sites (hepatic, mesenteric, cerebral, dermal veins) is characteristic of the thrombophilia of PNH. Indefinite anticoagulation is recommended following a thromboembolic event and thrombolytic therapy should be considered for acute hepatic vein thrombosis (Budd-Chiari syndrome). Pregnancy in a patient with PNH is complicated and requires careful management including prophylactic anticoagulation. To obtain a broad overview of the natural history, approaches to management, and outcome, the International PNH Registry was recently established.