This study is an open label, multi-center study of eculizumab, administered intravenously for 52 weeks to approximately 85 PNH patients. The study is designed to evaluate the safety of eculizumab in transfusion dependent patients with paroxysmal nocturnal hemoglobinuria (PNH) and to determine if the administration of this terminal complement inhibitor could provide a safe and effective substitute for CD59 function.