PURPOSE: It is difficult to make a distinction between lower urinary tract infection(UTI) and acute pyelonephritis(APN) during the acute phase of febrile UTI due to nonspecific clinical symptoms and laboratory findings, especially among young children. We measured the serum procalcitonin(PCT) in children with UTI to distinguish between acute pyelonephritis and lower UTI, and to determine the accuracy of PCT measurement compared with other inflammatory markers. METHODS: Serum samples were taken from children who admitted with unexplained fever or were suspected of having UTI. 51 children(mean 12.2+/-11.4 months) were enrolled in this study. Leukocyte counts, erythrocyte sedimentation rates(ESR) and C-reactive protein(CRP) were also measured. Renal parenchymal involvement was assessed by (99m)Tc DMSA scintigraphy in the first 7 days after admission. PCT was measured by immunoluminometric assay. RESULTS: PCT values were significantly correlated with the presence of renal defects in children with UTI(n=16)(5.06+/-12.97 microgram/L, P<0.05). However, PCT values were not significantly different between children with UTI without renal damage(n=18) and children without UTI(n=17). Using a cutoff of 0.5 microgram/L for PCT and 20 mm/hr for ESR, 20 mg/L for CRP, sensitivity and specificity in distinguishing between UTI with and without renal involvement were 81.3 percent and 88.9 percent for PCT 87.5 percent and 72.2 percent for ESR, and 87.5 percent and 55.6 percent for CRP, respectively. Positive and negative predictive values were 86.7 percent and 84.2 percent for PCT and 60.9 percent and 81.8 percent for CRP, respectively. CONCLUSION: In febrile UTI, PCT values were more specific than CRP, ESR and leukocyte count for the identification of patients who might develop renal defects.