Atypical hemolytic uremic syndrome (aHUS) is a rare syndrome characterized by microangiopathic hemolytic anemia, thrombocytopenia, and acute renal injury, which results from uncontrolled complement activation. Delayed diagnosis and treatment of aHUS may result in end-stage renal disease (ESRD) and an associated dependence on dialysis. In extreme cases, it may cause death due to multi-organ failure. Eculizumab, a humanized monoclonal antibody against C5, inhibits the formation of the terminal membrane attack complex and is used to treat aHUS. Here, we report a 46-year-old male patient who suffered from aHUS relapse, despite prior treatment with repeated plasma exchange and hemodialysis. Eculizumab therapy improved his hematologic findings without use of hemodialysis. (Korean J Med 2020;95:124-128)