Aztreonam (AZT) (2 g daily) was intravenously administered for 5 to 7 days to 65 patients with complicated urinary tract infections (UTI). Ninety one organisms were isolated from these patients' urine. Among them 66 organisms were eliminated following AZT administration. The elimination rate for Gram negative organisms was 94.7% and that for Gram positive organisms was 33.3%. Thirty one organisms appeared newly by the replacement of organisms. Among them, 12 organisms were E. faecalis. The efficacy rate of AZT in single infection and in mixed infection was 55.8% and 13.7%, respectively. The overall clinical efficacy rate of AZT in the present study was 41.5%. As side effects, skin rash and itching were observed in 3 patients and slight elevations in blood chemistry were observed in 10 patients. The results indicate that AZT is an effective drug for the treatment of patients with complicated UTI, especially those caused by Gram negative organisms.