aHUS is a rare condition (incidence of 2/1,000,000 in the United States) that results from uncontrolled activation of the alternate complement pathway and is characterized by microangiopathic hemolytic anemia, low platelet count (thrombocytopenia), the formation of intravascular platelet-rich thrombi, and multiorgan failure. Nearly 5% of all HUS cases are thought to be due to aHUS. There is both a familial (<20%) and a sporadic form of the disease. aHUS may rarely present as a potentially fatal complication after solid organ transplantation (SOT). Ischemia/reperfusion injury, use of calcineurin inhibitors (eg, tacrolimus, cyclosporine), and viral infections such as cytomegalovirus can precipitate aHUS after SOT. Other well-known precipitants for the sporadic form of atypical HUS include the following: HIV infection; cancer; organ transplantation; pregnancy; and the use of certain anticancer drugs, immunotherapeutic agents (eg, cyclosporine, tacrolimus), and antiplatelet drugs (eg, ticlopidine, clopidogrel).