Urinary levels of N-acetyl-beta-glucosaminidase (NAG) were measured in 147 consecutively enrolled children younger than 13 years of age with urinary tract infection to determine whether elevated levels were a predictor of urologic abnormalities. The children were classified as having cystitis if results of 0 or 1 of the following tests were positive and as having pyelonephritis if results of greater than or equal to 2 tests were positive: (1) temperature greater than 38 degrees C, (2) serum C-reactive protein greater than 1 mg/dL, (3) erythrocyte sedimentation rate greater than 25 mm/h, and (4) 1-deamino-8-D-arginine vasopressin-renal concentrating protein less than 810 mOsm/kg. Urinary NAG to creatinine ratios did not distinguish cases of cystitis from those of pyelonephritis. Urinary NAG was useful in identifying children with cystitis who had vesicoureteral reflux of grades II through V. Of 6 children with cystitis and vesicoureteral reflux, 5 had levels of NAG more than 1 SD above the mean, whereas of 75 children without vesicoureteral reflux, only 15 had such an elevation (P = .003). Of those children with a normal NAG level, 60 (98.4%) had normal radiologic evaluation results, and only 1 child (1.6%) had vesicoureteral reflux. Levels of NAG did not identify children with pyelonephritis who had vesicoureteral reflux. It is concluded that (1) urinary NAG is of no value in localizing the site of urinary tract infection, and (2) an NAG level within 1 SD of the mean in a child with cystitis indicates a low risk of urologic abnormalities, and radiologic evaluation may be omitted unless infection recurs.