Aztreonam (AZT, Azactam) was given to 176 patients with complicated urinary tract infections (UTI), of whom 110 cases satisfied with the "Criteria of UTI Committee for the Evaluation of Drug Efficacy in the UTI (3rd Ed.)", which was modified by adopting the midstream urine data in cases of women. Aztreonam was administered by drip infusion at a daily dose of 1 to 4 grams (mostly 2 grams) for at least five days long. The overall clinical efficacy rate of AZT in the 110 cases with the complicated UTI was 64%, estimated by the criteria cited above. The efficacy rate according to the infection type groupings was 66% for the 38 patients in the 1st group, 33% for the 3 in the 2nd group, 79% for the 19 in the 3rd group, 85% for the 21 in the 4th, 47% for the 19 in the 5th and 42% for the 12 in the 6th group. The disappearance rate of Gram negative bacilli was 87.1%. Especially 15 strains of Pseudomonas aeruginosa were eliminated out of 25 (60%) after AZT administration. Forty-six strains appeared after AZT treatment, 36 (76%) of which were Gram positive cocci. Subjective side effects and abnormal laboratory findings in a total 176 cases were: diarrhea in one, upper abdominal discomfort in one, increases in glutamic pyruvic transaminase in 8, increases in glutamic oxaloacetic transaminase in 5 and eosinophilia in 1. All of these disappeared after termination of the drug. AZT might therefore be a useful and safe antibiotic for the first choice use against complicated UTI.