The hemolytic-uremic syndrome (HUS) has been recognized for more than 45 years and consists of the combination of hemolytic anemia, thrombocytopenia, and acute renal failure. HUS occurs predominantly in children younger than 4 years of age. It is the most frequent cause of acute renal failure in children. The most common form of the syndrome (D+ HUS) occurs in healthy young children (>6 mo to <5 y of age) and is preceded by watery diarrhea that can evolve to hemorrhagic colitis. The diarrhea precedes the hemolysis and thrombocytopenia by 5 to 7 days; oliguria/anuria follows several days later. Although the pathogenesis is unknown, available evidence strongly suggests that endothelial cell damage is necessary. The outcome for most patients who have D+ HUS is favorable: 65% to 85% recover completely, 5% to 10% die (usually during the acute illness), recurrence is uncommon, and only a few patients slowly progress to end-stage renal disease (ESRD).