Atypical hemolytic uremic syndrome (aHUS) is a rare disease characterized by the 
triad of microangiopathic hemolytic anemia, thrombocytopenia, and acute renal 
failure. The term aHUS was historically used to distinguish this disorder from 
Shiga-toxin producing Escherichia coli (STEC)-HUS. Many aHUS cases 
(approximately 70%) are reportedly caused by uncontrolled complement activation 
due to genetic mutations in the alternative pathway, including complement factor 
H (CFH), complement factor I (CFI), membrane cofactor protein (MCP), 
thrombomodulin (THBD), complement component C3 (C3), and complement factor B 
(CFB). Mutations in the coagulation pathway, such as diacylglycerol kinase ε 
(DGKE) and plasminogen, are also reported to be causes of aHUS. In this review, 
we have focused on aHUS due to complement dysfunction. aHUS is suspected based 
on plasma ADAMTS13 activity of 10% or more, and being negative for STEC-HUS, in 
addition to the aforementioned triad. Complement genetic studies provide a more 
specific diagnosis of aHUS. Plasma therapy is the first-line treatment for 
patients with aHUS and should be initiated as soon as the diagnosis is 
suspected. Recently, eculizumab, a humanized monoclonal antibody against C5, was 
shown to be an effective treatment for aHUS. Therefore, early diagnosis and 
identification of the underlying pathogenic mechanism is important for improving 
the outcome of aHUS.