The clinical efficacy and safety of aztreonam were studied in 30 patients suffering from urinary tract infections caused by gram-negative bacteria. Aztreonam was given in a dose of 1 g i.m. at intervals of 12 or 24 hours. Duration of therapy was 2-8 days. Bacteria isolated in urine cultures were: Escherichia coli (60%); Proteus mirabilis (10%); Pseudomonas aeruginosa and Citrobacter sp. (7%); Proteus vulgaris, Serratia marcescens; Klebsiella pneumoniae and Pseudomonas species (3%). At completion of therapy and at follow-up all patients were bacteriologically and clinically cured. No significant side effects or toxicity were observed in all patients treated. It is concluded from these data that aztreonam is highly effective and well tolerated in patients affected either by upper or lower urinary tract infections.