Hemolytic uremic syndrome (HUS) is characterized by the triad of microangiopathic anemia, thrombocytopenia, and renal failure and is generally classified as diarrhea associated (D+HUS) and atypical or sporadic (D–HUS). The typical form is most commonly caused by E coli O157:H7 and is one of the most common causes of renal failure in children younger than age 2 to 3 years. Boys and girls are affected equally. Typically, there is a prodromal illness, usually gastroenteritis, although sometimes following upper respiratory infections. Gastrointestinal symptoms include abdominal pain, vomiting, and diarrhea, which may be bloody. Patients may have hepatitis, pancreatitis with endocrine and exocrine dysfunction. The abdomen is often tender. Central nervous symptoms include drowsiness, lethargy, irritability, and seizures. Oligoanuria with renal failure is common at presentation. There may be evidence of fluid overload, including hypertension and congestive heart failure. Most patients look pale, and petechiae and ecchymoses may be present. All patients have microangiopathic hemolytic anemia with negative direct and indirect Coombs. Peripheral smear shows fragmented and helmet-shaped red cells. Haptoglobin is low, and indirect bilirubin and plasma lactic dehydrogenase (LDH) are usually elevated. Platelets are usually diminished to <100,000. These patients have normal prothrombin and partial thromboplastin time. Urinalysis shows hematuria, proteinuria, and cellular casts. Acute renal failure is manifested with oligoanuria, raised BUN and creatinine, and other features of renal failure, including hyperkalemia, hypocalcemia, and metabolic acidosis. Gastrointestinal manifestation may be confused with intussusception, ulcerative colitis, and rectal prolapse or acute bacterial enterocolitis.