We determined the frequency and clinical significance of white blood cell (WBC) counts greater than or equal to 25,000/microliters in children presenting to an emergency department (ED) and defined a degree of leukocytosis which might be considered extreme in this setting. Records of all patients seen in the ED between February 1985 and December 1986 with WBC counts greater than or equal to 25,000/microliters were identified. Each patient was paired with the chronologically nearest patient with a WBC count between 15,000 and 25,000/microliters. Of the total WBCs obtained, 5.8% were greater than or equal to 25,000/microliters; only 1% were greater than or equal to 35,000/microliters. Eighteen percent of patients with counts greater than or equal to 25,000/microliters had a serious disease, and 6% had bacteremia. Twenty-six percent of patients with counts greater than or equal to 35,000/microliters had a serious disease, and 10% had bacteremia. On the basis of infrequency and severity of illness, we suggest that, in children presenting to a pediatric emergency department, WBC counts greater than or equal to 35,000/microliters be considered extreme leukocytosis.