Sixty-eight children with diarrhea-associated hemolytic uremic syndrome (D + HUS) were retrospectively examined to assess clinical variables associated with the combination of leukocytosis and hypocomplementemia. There was a statistically significant association between the white blood cell count (WBC) and the level of the third component of the complement system (C3). Children with both a low C3 and a high WBC were significantly younger and required hospitalization for a significantly longer period of time. Although there were also trends to increases in the presence of anuria and central nervous system complications and in the duration of anuria, elevated WBC, thrombocytopenia, dialysis, and hemorrhagic colitis in children with both an elevated WBC and a low C3, these changes did not achieve statistical significance. The presence of a low C3 and an elevated WBC may indicate a subset of children with D + HUS with a more severe episode.