Atypical hemolytic uremic syndrome is a rare life-threatening disease of unregulated complement activation. Untreated, the prognosis is generally poor; more than one-half of patients die or develop end-stage renal disease within 1 year. Atypical hemolytic uremic syndrome is characterized by thrombotic microangiopathy with evidence of hemolysis, thrombocytopenia, and renal impairment. This systemic disease affects the kidneys, brain, heart, lungs, gastrointestinal tract, pancreas, and skin. Acquired and genetic abnormalities of complement regulation may be identified in approximately 70% of patients. Plasma therapy is generally ineffective. Eculizumab blocks terminal complement activation, prevents complement-mediated organ damage, and is currently recommended as front-line therapy.