The complement system plays a central part in both innate and acquired immunity, but the contribution of complement activation to pathobiology is largely ancillary. An exception to the non-dominant role of complement in disease is the haemolytic anaemia of paroxysmal nocturnal haemoglobinuria (PNH). The intravascular haemolysis that is the clinical hallmark of PNH is a consequence of deficiency of the complement inhibitory proteins decay accelerating factor (DAF, CD55) and membrane inhibitor of reactive lysis (MIRL, CD59). Eculizumab is a humanised monoclonal antibody that binds and prevents activation of complement C5 and the subsequent formation of the cytolytic membrane attack complex of complement. Eculizumab inhibits the intravascular haemolysis of PNH, reduces transfusion requirements, stabilises haemoglobin concentration, and improves quality of life. Although chronic treatment with eculizumab increases the risk of infections with Neisseria meningitides, the drug is generally safe and well tolerated. But as is the case with other drugs developed for treatment of ultra-orphan diseases, eculizumab is expensive, and treatment must continue indefinitely because C5 inhibition does not affect the process (ie, clonal proliferation of haemopoietic stem cells with a mutant phosphatidylinositol glycan complementation class A [PIGA] gene) that underlies PNH. Moreover, due to the heterogeneous nature of the disease, treatment with eculizumab is not appropriate for all patients with PNH.