This study was designed to determine whether the measurement of interleukin (IL)-6 in urine is useful for distinguishing between acute pyelonephritis and lower urinary tract infection. This observational study was carried out at León Hospital (Spain) on 35 patients (ten boys) aged between 0 and 14 years with urinary tract infection. Urinary levels of IL-6 were determined with enzyme-linked immunosorbent assay (ELISA) at diagnosis and after recovery. Renal dimercaptosuccinate acid (DMSA) scan was performed on all patients to discard or confirm acute pyelonephritis. The mean urinary concentration [x +/- standard deviation (SD)] of IL-6 at diagnosis was 20.3 +/- 23.3 and 5.3 +/- 9.7 pg/ml in patients with acute pyelonephritis and lower urinary infection, respectively [95% confidence interval (CI): 2.6-27.4; p < 0.01]. Specificity for a value of IL-6 >15 pg/ml, was 94.1% (95% CI: 91.1-97.1). Positive predictive value for IL-6 >15 pg/ml was 87.5% (95% CI: 81.1-93.8). IL-6 was undetectable in the urine of both groups of patients at the time of recovery. Urinary levels of IL-6 are useful in differentiating between upper and lower urinary tract infection in children. In this clinical setting, a value >15 pg/ml is a strong indicator of acute pyelonephritis.