Atypical hemolytic uremic syndrome (aHUS) is a rare genetic life-threatening disease of chronic uncontrolled complement activation leading to thrombotic microangiopathy (TMA) and severe end-organ damage. Eculizumab, a terminal complement inhibitor approved for aHUS treatment, was reported to improve hematologic and renal parameters in 2 prior prospective phase 2 studies. This is the largest prospective study of eculizumab in aHUS to date, conducted in an adult population.