Urinary excretion of NAG was determined in 32 patients aged four months to 15 years with urinary tract infection (UTI). Level diagnosis was made by means of commonly accepted clinical and laboratory criteria. Pathological enzymuria was present in all 15 patients with pyelonephritis while 12 of 14 children with cystitis had a normal urinary NAG excretion. Two patients with cystitis and 3 patients with questionable level diagnosis had elevated urinary NAG levels. According to these data the level diagnosis of UTI was changed in 16% of the patients by including the NAG values. Follow-up studies of nine patients showed a significant decrease of initially elevated urinary NAG levels after a 10-days course of antibiotic treatment and the values were within the normal range in 8 of 9 patients. Determination of urinary NAG excretion seems to be of definite value as an additional parameter for level diagnosis of childhood urinary tract infection.