Both hemolytic uremic syndrome (HUS) and thrombotic thrombocytopenic purpura (TTP) are characterized by thrombotic microangiopathy (TMA), affecting mainly the kidney and brain, respectively. Diagnosis of HUS or TTP has been complicated by the fact that these disorders share several clinical characteristics, and by the dearth of knowledge regarding the pathogenesis of TMA. Advances in the identification of pathogenic features--deficiency of the metalloprotease ADAMTS13 in TTP and association of mutated complement proteins with atypical HUS--have gone some way towards improving clinicians' ability to distinguish between the two diseases. Here, we pose the following question: is it important to patient management that HUS be distinguished from TTP? By discussing what is known about the pathogenesis, clinical features and treatment of these two conditions we address this question, and propose a new nomenclature for TMA.