Atypical hemolytic-uremic syndrome (aHUS) is a rare form of thrombotic microagiopathy caused dysregulation of the alternative pathway of the complement resulting in tissue. In aHUS, activation of the alternative pathway of the complement is in an aberrant way directed against endothelial cells and blood cells. This is either due to a mutation in a complement factor, most commonly factor H, or an autoantibody against a complement regulator. In some patients the underlying disorder is not identified despite thorough examinations. Typical aHUS-patients have acute kidney injury and microangiopathic hemolysis and, to a varying degree, disturbances of other organs. An effective inhibitor of the final product of complement, eculizumab, has revolutionized the treatment of these patients.