Objective: It is difficult to make a distinction between acute pyelonephritis and lower urinary tract infection due to nonspecific clinical symptoms and laboratory findings. We measured the spot urine β2-microglobulin in children with urinary tract infection (UTI) to distinguish between acute pyelonephritis and lower UTI. We compared the accuracy of urine β2-microglobulin measurement with other inflammatory markers. Methods: We studied 83 children (mean, 86± 44.9 months) who suspected of having UTI. Leukocyte counts, erythrocyte sedimentation rates (ESR) and C-reactive protein (CRP), β2-microglobulin were measured. Renal parenchymal involvement was evaluated by 99mTc dimercaptosuccinic acid scintigraphy in the first 7 days after admission. β2-microglobulin was measured by radioimmunoassay. Results: Urine β2-microglobulin values were correlated with the presence of renal defects in children with UTI (n= 22) (0.98± 0.24 μg/mL, P< 0.05). Using a cutoff of 0.4 μg/mL for spot urine β2-microglobulin and 20 mm/hr for ESR, 2.6 mg/L for CRP, sensitivity and specificity between UTI with and without renal involvement were 78.7% and 90.1% for spot urine β2-microglobulin, 77.2% and 90.1% for spot urine β2-microglobulin/creatinine (Cr), 77.2%, 68.8% for ESR, 86.3%, 68.8% for CRP, respectively. Positive and negative predictive values were 72.7%, 90.1% for spot urine β2-microglobulin, 73.9% and 91.6% for spot urine β2-microglobulin/Cr, and 57.5%, 94% for CRP, respectively. Conclusion: In febrile UTI, spot urine β2-microglobulin and β2-microglobulin/Cr values were more specific than CRP, ESR, and leukocyte count for determine the renal defects.