BACKGROUND: Paroxysmal nocturnal hemoglobinuria (PNH) is an acquired clonal disorder characterized by chronic complement-mediated hemolysis. Eculizumab, a humanized monoclonal antibody against the terminal complement protein C5, potently reduces chronic intravascular hemolysis. We tested the clinical efficacy and safety of a 24-week treatment with eculizumab in 6 Korean patients with PNH. METHODS: We enrolled 6 patients with PNH who had clinically significant hemolysis. Eculizumab was administered intravenously at 600 mg/week for the first 4 weeks followed by 900 mg at week 5 and 2nd weekly thereafter. RESULTS: Three men and 3 women with a median age of 39.5 years (24-61 years) were enrolled. The median duration of PNH was 11 years (6-25 years). Hemolysis occurred in all patients [median lactate dehydrogenase (LDH) level, 7.95 times the upper limit of the reference range of LDH]. All patients treated with eculizumab had a rapid and sustained reduction in the degree of hemolysis. RBC transfusion requirements for 3 months were decreased from 0-12 units (median requirement, 1.5 units) to 0-6 units (median requirement, 0 units). Improvement in fatigue was noted in 4 patients. Further, 5 patients who had been receiving corticosteroids either reduced the dose or discontinued therapy. No significant adverse events related to eculizumab therapy were observed. CONCLUSION: These results show that eculizumab reduces the degree of intravascular hemolysis, reduces or eliminates the requirement of RBC transfusion, and improves anemia and fatigue in patients with PNH. Eculizumab is an effective and safe option for treating Korean patients with PNH.