Paroxysmal nocturnal hemoglobinuria (PNH) is a rare clonal blood disorder that manifests with hemolytic anemia, bone marrow failure, and thrombosis. Many of the clinical manifestations of the disease result from complement-mediated intravascular hemolysis. Allogeneic bone marrow transplantation is the only curative therapy for PNH. Eculizumab, a monoclonal antibody that blocks terminal complement activation, is highly effective in reducing hemolysis, improving quality of life, and reducing the risk for thrombosis in PNH patients. Insights into the relevance of detecting PNH cells in PNH and other bone marrow failure disorders are highlighted, and indications for treating PNH patients with bone marrow transplantation and eculizumab are explored.