We report the case of a 12-year-old boy with fatal enteropathic hemolytic-uremic syndrome who developed excessive neutrophilia in the course of his disease, his leukocyte count exceeding 200,000/mm3. Neutrophilia, as it was observed in this case, is an extreme manifestation of a phenomenon, that is commonly observed in hemolytic-uremic syndrome [Salzmann et al. 1991]. Neutrophilia is suspected to be correlated with a bad prognosis [Walters et al. 1989], but further explanation of this phenomenon is needed. Other underlying diseases related with neutrophilia, especially hematologic malignancies, could be ruled out by far. We examined endogenous G-CSF serum concentrations of the HUS patient from day 6 to 13 in the course of the disease. The assayed concentrations were found to be elevated in the first two samples taken (peak level 340 pg/ml). In the samples taken after plasmapheresis had started, G-CSF concentrations were not found to be elevated. The peak of neutrophilia was reached short before death on day 13 of the disease. We also measured the serum G-CSF concentrations of 28 children aged 3 months to 12 years, who were treated with various infectious and noninfectious diseases in our hospital. In none of the examined samples was there a G-CSF serum concentration exceeding 50 pg/ml. The reported case shows evidence that the commonly observed coincidence of leukocytosis and HUS may reflect the role of G-CSF (and other cytokines) in the inflammatory process underlying the HUS.