Clinical effect was studied for aztreonam (AZT) on 2 patients being treated for malignant tumors. One-two grams of AZT was administered b.i.d. by intravenous drip infusion for 6-7 days. In case 1 with pyometra, there were excellent improvements in WBC, CRP and fever pattern. Unfavorable BUN and S-Cr. values were found, but these were considered to be side effects due to cisplatin. These improved finally, and clinical effect was also excellent. Case 2, a urinary tract infection + prophylactic postoperative administration case, was treated in combination with another drug presuming mixed infection and AZT was shown to be effective clinically and overall. No side effects in cases 1 or 2 were considered to be due to AZT.