Plain Language Summary Paroxysmal nocturnal hemoglobinuria (PNH) is a rare blood disorder characterized by the destruction of red blood cells (hemolysis) within blood vessels. In addition to hemolysis, patients with PNH are susceptible to life-threatening blood clots (thromboses). Eculizumab and ravulizumab are types of treatments for PNH, called C5 inhibitors. In the blood, these treatments bind to C5 protein and prevent the destruction of red blood cells, reducing the symptoms and complications of PNH. Both treatments are approved for use via intravenous (through the vein) administration. Ravulizumab is also approved in the USA for use via subcutaneous (under the skin) administration. This study compared subcutaneous ravulizumab with intravenous ravulizumab in patients with PNH who had previously been treated with eculizumab. During the initial treatment period of 71 days, 90 patients received subcutaneous ravulizumab and 46 received intravenous ravulizumab. Following this period, all patients received subcutaneous ravulizumab. At day 71, the amount of ravulizumab in the blood of patients taking subcutaneous ravulizumab was no less than in patients taking intravenous ravulizumab and was maintained over 1 year of treatment. Efficacy measures (how well it works) remained stable in patients taking subcutaneous ravulizumab for 1 year and side effects were comparable with those of intravenous ravulizumab. Patients reported more satisfaction with subcutaneous ravulizumab than intravenous eculizumab, as assessed by the Treatment Administration Satisfaction Questionnaire. This study showed that patients with PNH can switch from intravenous eculizumab or ravulizumab to subcutaneous ravulizumab without loss of efficacy. Subcutaneous ravulizumab provides an additional treatment choice for patients with PNH.