A multicenter trial involving several urologic units in Italy provided pooled data on 1,427 patients with urinary tract infections to evaluate the efficacy and safety of aztreonam, a new monobactam antibiotic. Microbiologic and clinical data were collected methodically in all cases. A majority of the patients (79.9%) were hospitalized during the study period, an associated pathology was noted in 29.1%, and 16.7% were receiving additional therapy. Aztreonam was administered by different routes and in different dosages according to the severity of the pathology. At the end of treatment, 93.6% of the patients showed a positive microbiologic response. Eradication percentages of the 6 main pathogens determined from cultures taken within the seventh day after the end of treatment were as follows: 93.9% for Escherichia coli (n = 415), 86.3% for Pseudomonas sp. (n = 207), 91.6% for Proteus sp. (n = 192), 89.8% for Providencia sp. (n = 59), 96.2% for Klebsiella sp. (n = 56), and 98.1 % for Serratia sp. (n = 56). Aztreonam was well tolerated. Of the 1,427 patients evaluated for safety, only 54 (3.8%) reported 55 adverse reactions, necessitating the withdraw al of therapy in 5 (0.2%) instances.