Urinary interleukin-6 (UIL-6) and urinary interleukin-8 (UIL-8) concentrations were measured by immunoassay in 39 and 34 patients respectively, hospitalized with febrile urinary tract infection (UTI), and in 37 and 32 age-, race- and sex-matched febrile control children respectively, with negative urine cultures. UIL-6 and UIL-8 concentrations, measured in picograms per milliliter and corrected for creatinine, were compared with clinical and laboratory indicators of inflammation and bacterial virulence factors of Escherichia coli. Median UIL-6 concentrations at the time of admission were 397 pg/ml (range 0-65,789 pg/ml) in the 37 patients compared to 0 pg/ml (range 0-473.8 pg/ml) in the 37 controls (P < 0.0001). Median UIL-8 concentrations at the time of admission were 5809 pg/ml (range 0-347,368 pg/ml) in the 32 patients compared to 0 pg/ml (range 0-2231 pg/ml) in the 32 controls (P < 0.0001). UIL-6 and UIL-8 concentrations were lower (P < 0.0001 for UIL-6 and P = 0.0005 for UIL-8) in follow-up urine samples from UTI patients, obtained 48 h after the initiation of antibiotic therapy. UIL-6 and UIL-8 concentrations were statistically significantly correlated with urine white blood cells (WBC). UIL-8 concentrations were elevated in patients with E. coli organisms producing hemolysin. UIL-6 and UIL-8 are elevated in children with febrile UTI and decrease in response to antibiotic therapy. Magnitude of UIL-8 response is associated with hemolysin production, a bacterial virulence factor of E. coli. UIL-6 and UIL-8 concentrations are statistically correlated with urine WBC. UIL-6 and UIL-8 may be mediators of inflammation in children with febrile UTI.