BACKGROUND: Urinary tract infection (UTI) is a common problem in children. Because clinical findings and commonly used blood indices are nonspecific, the distinction between lower and upper urinary tract infection cannot be made easily in this population. However, this distinction is important because renal infection can induce parenchymal scarring. The objective of this study was to determine the accuracy of procalcitonin (PCT) compared with C-reactive protein (CRP) rapid tests to predict renal involvement in children with febrile UTI. METHODS: PCT and CRP were measured in the blood of children admitted to the emergency room with fever, signs and symptoms of urinary tract infection and/or a positive urine dipstick analysis. Renal parenchymal involvement was assessed by a 99mTc-labeled dimercaptosuccinic acid renal scan in the acute phase of infection in all children. Sensitivity, specificity and likelihood ratios were determined for both tests. RESULTS: Fifty-four children with a proven urinary tract infection were enrolled: 63% had renal involvement; and 37% had infection restricted to the lower urinary tract. No difference was found for age, sex and total white blood cell count between the groups. The calculated likelihood ratios of procalcitonin and C-reactive protein rapid tests were between 3.8 and 7 and 1.5 and 2.8, respectively. A positive PCT value predicted renal involvement in 87 to 92% of children with febrile UTI, compared with 44 to 83% using CRP values. CONCLUSIONS: A rapid determination of procalcitonin concentration could be useful for the management of children with febrile UTI in the emergency room.