This study will evaluate the safety and effectiveness of the experimental drug eculizumab for long-term treatment of paroxysmal nocturnal hemoglobinuria (PNH). PNH can cause premature destruction of red blood cells, resulting in anemia that requires blood transfusions. Patients may be at high risk of life-threatening blood clots in their veins. Some patients can develop bone marrow failure or aplastic anemia. Other symptoms include urine discoloration, stomach pain, difficulty in swallowing, tiredness, and poor quality of life. Men may have problems getting or maintaining an erection.

Eculizumab is a monoclonal antibody that may improve the survival of red blood cells in patients with PNH.

Patients with PNH who have fully completed the SHEPHERD study or have completed the TRIUMPH study through at least visit 18 may be eligible for this study.

Participants from the SHEPHERD study will receive infusions of 900 mg of eculizumab through a needle in a vein every other week for about 53 doses.

Participants who received placebo in the TRIUMPH study will receive 600 mg of eculizumab once a week for four doses, followed by 900 mg of drug a week later for one dose, and then 900 mg of drug every other week for another 50 doses. Patients who received eculizumab in the TRIUMPH study will receive 900 mg eculizumab every other week at visits 1 and 3 and placebo every other week at visits 2 and 4, followed by 900 mg of drug every other week for about 53 doses. Visits are scheduled every week for the first five visits and then every other week until the end of the study.

At every treatment visit, each patient updates health status, transfusion record and medication use, reviews their laboratory results from the preceding visit, and receives an infusion of eculizumab. At selected visits, they have a full physical examination, complete a questionnaire about how the illness affects their everyday life, provide blood and urine samples, and undergo an electrocardiogram. Blood is collected more often if clinically indicated.

All patients receive a prescription for an antibiotic with instructions on signs and symptoms of Neisseria meningitides infection. N. meningitides is a common bacterium that can cause symptoms, possibly including life-threatening meningitis, in susceptible people, including people who take eculizumab.

Participants are re-vaccinated against this bacterium when more than 2.5 to 3 years have passed since their previous inoculation.

Participants who leave the study early are followed for an additional 16 weeks with six visits to update their health status, transfusion record, and medication use, have their temperature, heart rate, and blood pressure measured, and provide a blood sample.