This study analyzed the interleukin 6 (IL-6) response in 114 children with suspected urinary tract infection (UTI). Urine and serum samples were obtained at the time of enrollment. There were 90 children with UTI, 41 with and 49 without a temperature ≥38.5°C. The remaining 24 children did not have bacteriuria; 11 were febrile and 13 were not. The urinary IL-6 concentrations were higher in the children with UTI (mean, 129 units/ml) than in the children without bacteriuria (mean, 7 units/ml,P< 0.01). In contrast the serum IL-6 did not differ between children with or without UTI or between children with or without a temperature ≥38.5°C. The urinary IL-6 response was higher in children who were infected with P fimbriatedEscherichia colithan in other children with UTI (P< 0.05). There was a correlation of urinary IL-6 with the degree of proteinuria, hematuria and urinary leukocyte counts (P< 0.001,P< 0.05,P< 0.05, respectively) but not with serum IL-6, CRP or temperature, and of serum IL-6 to C-reactive protein (P= 0.053) and renal concentrating capacity (P< 0.05). The results demonstrate that infections of the urinary tract activate an IL-6 response in children and that the magnitude of the IL-6 response is influenced by the properties of the infecting strain.