Hemolytic uremic syndrome (HUS) is the main cause of acute renal failure in children, and the primary diagnosis in 4.5% of children in chronic renal transplantation therapy. HUS is characterized by acute renal failure, hemolytic anemia and thrombocytopenia. The typical form of HUS follows gastrointestinal infection by enterohemorrhagic Escherichia coli. 5% of all HUS cases show an atypical or recurrent course. Mutations in complement regulatory proteins play an important role in the pathogenesis of atypical HUS and in the outcome after renal transplantation. These patients have a very high risk of graft loss due to recurrence of HUS or thrombosis. Patients with HUS and no evidence of EHEC infection should be fully investigated for the known complement disorders and for autoantibodies against factor H. An accurate diagnosis of HUS based on the latest knowledge of complement dysregulation should help in predicting the risk of graft failure. New therapies are emerging and give hope for better future treatment of this severe disease. Copyrigth © Sociedad Iberoamericana de Información Científica (SIIC), 2010.