Atypical hemolytic uremic syndrome (aHUS) is an extremely rare, life-threatening, progressive disease that frequently has a genetic component. In most cases it is caused by chronic, uncontrolled activation of the complement system, a branch of the body’s immune system that destroys and removes foreign particles. The disease affects both children and adults and is characterized by systemic thrombotic microangiopathy (TMA), the formation of blood clots in small blood vessels throughout the body, which can lead to stroke, heart attack, kidney failure, and death. The complement system activation may be due to mutations in the complement regulatory proteins (factor H, factor I, or membrane cofactor protein), or is occasionally due to acquired neutralizing autoantibody inhibitors of these complement system components, for example anti–factor H antibodies. Prior to availability of eculizumab (Soliris), an estimated 33–40% of patients died or developed end-stage renal disease (ESRD) (despite the use of supportive care, e.g. plasmapheresis) with the first clinical bout of aHUS. Including subsequent relapses, a total of approximately two-thirds (65%) of patients died, required dialysis, or had permanent renal damage within the first year after diagnosis despite plasma exchange or plasma infusion (PE/PI).