Azthreonam, the first monobactam, was given to 40 patients with urinary tract infection. Patients included 27 females, aged 17 to 77 years. UTI was complicated cystitis in 10 patients, pyelonephritis in 11, and prostatitis in 19. The following bacteria were recovered from urine: 12 E. coli, 1 Levinea , 3 Proteus mirabilis, 7 Klebsiella, 14 Serratia and 14 Pseudomonas. MICs of azthreonam ranged from 0.0035 to 16 micrograms/ml (mean 0.12 microgram/ml). Azthreonam was given intramuscularly, as monotherapy, in a daily dosage of 2 g, in two divided doses, for 10 to 29 days (mean: 28 days). Follow-up was at least 4 weeks after completion of treatment. Therapeutic results were as follows: 8 cures and 2 failures by relapse in 10 cystitis , 6 cures and 5 failures by relapse in 11 pyelonephritis , and 12 cures and 7 failures by relapse in 19 prostatitis . General and local tolerance were excellent. There were no hematologic or renal side effects. Transaminases SGOT and especially SGPT increased transiently in 7 patients and returned to normal after treatment was discontinued; premature withdrawal was needed in only one case.