Until recently, atypical hemolytic uremic syndrome (aHUS), conventionally defined in the pediatric literature as a syndrome of the triad of renal failure, microangiopathic hemolytic anemia, and thrombocytopenia without a prodrome of hemorrhagic diarrhea, has received little attention in adult practice because the patients are commonly given the diagnosis of thrombotic thrombocytopenic purpura (TTP) or TTP/HUS and treated as TTP with plasma exchange, augmented in refractory cases with rituximab and sometimes even splenectomy.