Hemolytic uremic syndrome (HUS) is a triad of microangiopathic hemolytic anemia, 
thrombocytopenia, and acute renal failure. The atypical form of HUS is a disease 
characterized by complement overactivation. Inherited defects in complement 
genes and acquired autoantibodies against complement regulatory proteins have 
been described. Incomplete penetrance of mutations in all predisposing genes is 
reported, suggesting that a precipitating event or trigger is required to unmask 
the complement regulatory deficiency. The underlying genetic defect predicts the 
prognosis both in native kidneys and after renal transplantation. The successful 
trials of the complement inhibitor eculizumab in the treatment of atypical HUS 
will revolutionize disease management.