Atypical hemolytic uremic syndrome (aHUS) is a rare form of thrombotic microangiopathy that associates, in 70% of cases, with genetic or acquired disorders leading to dysregulation of the alternative pathway of complement. Autoantibody directed against Factor H causes at least 6% to 10% of aHUS cases, but only a few clinical reports are available. Here, we describe the clinical, biologic, genetic features, treatment, and outcome of 45 patients who presented with aHUS associated with anti-FH autoantibody. We found that this form of aHUS primarily affects children between 9 and 13 years old but it also affects adults. It presents with a high frequency of gastrointestinal symptoms and with extrarenal complications and has a relapsing course. Activation of the alternative pathway of complement at the onset of disease portends a poor prognosis. Early specific treatment may lead to favorable outcomes. These data should improve the recognition and diagnosis of this form of aHUS and help identify patients at high risk of a poor outcome.