Clinical studies of aztreonam (AZT) were performed in 10 pediatric cases. One transient pyuria case with 10(3)/ml E. faecalis detected in urine was excluded from clinical evaluation, because the presence of infection was unclear. Results were as follows: AZT was effective on 1 patient with meningitis (causative organism: H. influenzae), who was treated with 41.7 mg/kg 4 times a day. Results of administration of 58.1-78.9 mg/kg 3 or 4 times a day by intravenous injection for 1 E. coli sepsis-and-pyelonephritis complication case and 7 pyelonephritis cases (causative organisms: E. coli in 1, E. coli + E. faecalis in 1, E. faecalis in 1, P. aeruginosa in 3 and unknown in 1) were excellent in 4, good in 2 and poor in 2 cases. The pathogens of the 2 poor cases were E. faecalis and P. aeruginosa, respectively. Six of the pyelonephritis cases had vesicoureteral refluxes as an underlying condition. Clinical and microbiological effects of AZT were considered to be closely correlated with its MIC values. No side effect was recognized. Though abnormal laboratory findings were obtained in 4 cases, including elevations of GOT X GPT in 2 cases, GPT elevation in 1 case and plateletcount increase in 1 case. All of these abnormalities were minor and transient. The serum concentrations of AZT for a two-month-old patient with pyelonephritis were 65, 50, 35, 22.8 and 12.4 micrograms/ml at 1/2, 1, 2, 4 and 6 hours, respectively and T1/2 was 2.42 hours after injecting AZT 20 mg/kg by intravenous injection.(ABSTRACT TRUNCATED AT 250 WORDS)